During the summer, you can't see the northern lights even in Finland, and in Helsinki it is bright almost around the clock.

As you mentioned, for example, accommodation and meals in Finland are more expensive than in Hungary. Both options are great and the decision is yours. If you come to Finland, I advise you to travel further north than Helsinki, Finland is a long country.

East Europe Traintravel is quite cheap often standard tickets (or Saver fares like “Sparpreis”) are cheaper than a Interrail pass esspecially if you have a fixed route with a fixed shedule the Interrailpass usually doesn´t worth the money :)

If you wanna stay 100% flexible i recommend to comine a flex Interrrailpass and standard tickets :)

To save some money i would adjust your East European trip a bit :)

For example use your Interrail outbound trip or German Sparpreis from 39€ on the Route Germany - Lubljana (from Frankfurt starts daily a direct train via Stuttgart - Munich - salzburg - Klagenfurt)

Lubljana - Budapest cheap fares can be found from just 15€ Slovenian Rail offer and a Interrailtravelday usually worths 25-35€ (depends on the purchased pass) 

Maybe add even Zagreb to your trip Slovenian Rail offer starts from 9€ to Zagreb

From Budapest you can use the scenic Eurocity Route along the Danube to Bratislava or use the cheap Regiojet trains (2 depatures per day) From Budapest via Vienna to Prague till Vienna (Regiojet have often quite cheap fares even last minute and it worths to compare it with Interrail and the offers of the National Railoperators (Ceske Drahy, Hungarian Rail (MAV - Start), Austrian Rail (ÖBB) or  Slovak Rail (ZSSK))

Regiojet starts on the Route till Vienna from just 9€ in low Cost (Still refundable till 15mins before depature)  Regiojet Booking website

Vienna - Bratislava (just one hour) can be done for 10€ Standard fare or as daytrip from Vienna with the special Bratislava Ticket 16€ includes even public transport in Budapest for the day ;)

Vienna or Bratislava to Prague once more REgiojet with fares from just 16€ often even last minute.

Prague - Krakow you can use twice per week a train of LeoExpress (another cheap railcompany from Czech Republic) or use maybe here one of your Interrail days or other cheap tickets from Ceske Drahy)

Krakow - Warsaw check for cheap options of PKP (Polish Rail) and other companies often even lastminute fares from just 16€ :)

Warsaw - Germany you could use your Inbound journey :)
Means according my suggestion you just used 2 -4   Traveldays of a flexipass :D
Germany - Lubljana
Budapest - Bratislava (optional)
Prague - Krakow
Warsaw - Germany :)

And usually you always find cheap hostels if you book 2-3 days in advance but if you wanna prebook you can look for Hostels that offer discounts for Railpass Holders on the website of Eurail and Interrail you can find a huge pass benefit´s section that shows where you get discounts with a Interrailpass Pass Benefits

I know´s its a long text and maybe a bit complicated to understand but don´t hesitate to ask here  :)
 

According your Scandinavia trip i recommend to explore even the far north :)

Here Interrail worths more as the tickets can be quite expensive

For example Germany - Denmark - Göteborg - Oslo - Trondheim - Östersund - Galliväre - (Narvik/Abisko/Kiruna) - then via the north Lulea - Happaranda/Tornio to Kemi / Rovanemi and then to Helsinki :)

The Route Östersund - Galliväre known under Inlandsbanan is a nice full day joruney and i recommend to book asap as the Route is very popular an often sold out (Route is only Service between end June - Early August)

From Helsinki by Ferry maybe with a small detour via Tallinn to Stockholm and finally from Stockholm by train back to Germany in summer direct Nighttrain from Stockholm to Berlin via Hamburg and ontop a second Nighttrain just till Hamburg :)

I did back in 2019 the ferry detour from Helsinki via Tallinn to Stockholm as the Ferry from Tallinn was cheaper as the direct ferry from Helsinki to Stockholm On these ferries you get a discount with Interrail ;)

Seewulf has stolen my comment already-would have written much the same.

Also note that HOStels with discount are often the more expensive ones, in those east-EUR cities-in normal times-there are dozens and more of cheap hoStels, simply use a general site like booking.

Scandinavia is a little cheaper relatively n their summer holidays, as there are more offers due to lack of bisnis people then.

Oh-one of the schönere Sachen re IR is that you can also stop in smaller towns, make easy daytrips-there is no need to simply count off the capitals. F.e. Bratislava is hardly worth it (IMHO)-visit the Tatra for nature in Slovakia! You could even make a sneak short trip into Ukrayna -from HUngary via Chop to Lviv-then back to POland. Just pay separate for bus or train going there. You need a full passpt, but nothing further. Much more exotic out of the EU!

Recommend the yellow trains of RegioJet (they accept also IR-but much cheaper to simply book advance) if they serve your route.

Oh-and do not forget that in East-EUR very often BUSes are cheaper, easier, quicker as trains on what seems like difficult routes. Esp. germans seem to think that es alles mit bahn sein sollte.

Wow! Thank you all for your detailed answers. You surprised me. 

I always thought the Interrail Pass would be the cheaper alternative to regular tickets. But then I’m going to look at it in more detail. 

Thank you!

Interrail is not always the cheapeast but it´s the best mix of money saving and staying flexible Thats why i combine often a Flexipass with cheap individual tickets :)

Saverfares “Sparpreise” are often cheaper but not exchangable or refundable and maybe have to be booked well in advance

Standard Tickets can often be bought last minute and are maybe exchangable or even refundable but can be quite expensive (depending on the Country, the Route, the travelcomfort and ofcourse the traincategory )

Hey, 

In Summer 2019 I was in Scandinavia and it was my first trip with Interail and alone. I started in Germany and go to Frederickshavn because I wanted to see the Oslofjord by ship. From Oslo I went to Bergen, than (over Oslo) over night to Trondheim. After the next Nighttrain I traveled in Fauske (near Bodo) with the Bus to Tromso and the Nordkap. There I got down to Finland with a Bus too (north of the Artic Circle are only Buses) to the Inari lake and Rovaniemi (Father Christmas!). Then I could take the train to Helsinki and to Savonlinna. From Turku I took the ferry to Stockholm on day for 7€ because of the Interail Pass. Over Lund I found my way back to Germany.

I booked only the hostel in Oslo before my journey. I didn`t have to sleep outside, but this wouldn`t be a problem in Scandinavia, because you can sleep wereever you want (not private ground) → „Jedermannsrecht“

Yes, Scandinavia isn`t cheap, but if you buy your food in the supermarket and don`t go (to often) to restaurants, it`s almoust like the prices in Germany.

Enjoy your planing and have a blessed journey

Just one small addition: Bring your own bedsheets if you go to Scandinavia because 95% of hostels in Scandinavia will charge extra for bedsheets! So to the already expensive price per night you can add another ~7 Euro for the bedsheets if you don’t have them by yourself….