The cheapest tickets appear about 3 months before your date of travel (supersparpreisen). If you plan to travel around easter holidays and long weekends and want absolute flexibility with your trains, a pass is probably the better choice. Especially if you take normal hourly regional trains (doing the exact same route as the Bernina express, but no need for extra reservations). 

Maybe you can scare a discounted interrail pass in the coming weeks/months? Best subscribe to the newsletter to keep posted, last years there were some black friday actions, and sometimes there were discount actions around february.

In General Switzerland is a tad bit expensive for ordinary tickets, so a pass is usually good value. But Rhätische bahn (who operates the trains to/from Tirano) every now and then does an interesting action as well, offering day tickets for the region at reduced prices.

For your plans it’s going to be tight, I’lld say to compare prices somewhere in january and compare well between pass, normal tickets and possible reservation cost. If you can get a discounted pass, try to buy it, and maybe add an extra scenic train day in Switzerland (e.g. Oberalpass, or another part of the Glacier express route)?

But again, I think it’s gonna be tight price wise and not a super big difference.

There is a way to get a single Supersparpreis ticket all the way to Tirano, including an overnight stop in Chur. You just need to put the correct amount of time for a stopover into the search on DB.

If you’re only doing Germany-Chur-Tirano and return, then that will be the cheapest option, if you can get a good Sparpreis.

Seat61 (scroll down bit to The clever way to buy Bernina Express tickets on this link) suggests they start from €29.99 each way (€39.99 in First Class) from southern Germany. 

Where will you start from in Germany?