Milan to Tirano to catch the Bernina Express to St. Moritz and return with Viator

Why pay a middle man for something you can easily do on your own.
A round trip ticket from Milano Centrale to Tirano is 23 EUR. A round trip from Tirano to St Moritz is 64 CHF. At current exchange rates, that is about $87 US total for the 4 train rides. If you ride the actual Bernina Express train, the is a 10 CHF "reservation fee" each way, about $10 US each way.

Normally I'd be with you Sam, but when you throw in having a guide and all the arrangements taken care of for you, on top of your costings, then the organised tour seems better value. For me it would the question of, "do I want to be a part of a group?"