If you're only doing London-Berlin and return, then compare with normal tickets. Deutsche Bahn have an offer that combines their Saver ticket with a cheaper Eurostar ticket if you book them together.

Here an example for @rvdborgt  suggestions 

It’s quite a long ride (about 10 hours one way) and if you do use Eurail be aware you’ll need to spend an additional €30 per trip on the Eurostar (€60 for the return) for the compulsory passholder seat reservations. Looking at the Monday 16th, there also seems to be limited availability: only the 08:16 and 09:01 from London currently have passholder seats available, and those might run out soon. No seat reservations are required on ICE services between Berlin and Brussels, and there are seats  available on the Friday Eurostar service.

I checked this for the given dates but didn’t get the DE connect saver ticket. It’s also pretty last minute, so it is pretty expensive: a regular return ticket costs €465.80. Using a pass is definitely cheaper (€221 for a 4-day in a month Eurail pass + €60 for the reservations).

The question is if there are still seats for Eurailers on these dates for Eurostar trains as the seats are limited :) 

Before buying the pass check here if there are still seats for Railpass user https://www.b-europe.com/EN/Booking/Pass#TravelWish

I checked it, though it wasn’t very clear. There are no seats available for the 13:01 Eurostar leaving London on the Monday, only for those at 08:16 and at 09:01. On the friday (Brussels-London) there are passholder priced seats available on more trains:

I had however missed posting the link, so thanks for adding that! Be sure to check this as shortly before buying your pass as possible, if you decide to go through with it, and then once you have your pass immediately book the reservations on the Eurostar.

You can book pass holder seats before you buy a pass.

ugh seems like trains arent the cheapest option, i had activated my eurail card on the 3rd but didnt end up using it so now im stuck trying to use it this month but doesnt seem like any option is budget friendly

It is very unwise to activate your pass in advance, certainly before making any mandatory reservations you may need, e.g. for Eurostar.

Now that you have the pass, you can still book Eurostar reservations (but be quick) and use your pass.

Fridays and sundays are very popular Eurostar days. If you would be able to move your travel plans, you probably get a whole lot more of options. But the long ride time, will get you to only enjoy Berlin for two full days. It's a big city with a lot of neighbourhoods. Not worth it for only two rushed days. Berlin deserves more time. Better look for a destination a bit closer. 

A lot of reservation free alternatives exist, but not Eurostar alas.

But on the other hand, as an American, you can enjoy the pass in the UK itself no? (Dunno if you have other destinations planned)

With some pass days over, you have immense flexibility and avoid the sometimes absurd ticket prices in the UK. Nearly every UK train is included the pass. Reservations are free. UK has plenty to offer too and as a lovely interrail/Eurail country if you want to make good use of your pass! 

If I read it correctly you are looking for London to Berlin and back. If so there are 3 Eurostar trains out on Friday morning to Brussels with pass availability at 30 euro. There are 2 back on Monday evening with availability. Obviously the onward ICE trains are optional reservations and readily available for a few euros from Dbahn.