As I think about it, I wonder if selling a Monster Manual really makes sense on a VTT, at least as the primary way to get monster tokens to dms.
It seems more logical to focus on making dming subscription-based: you pay your $15 a month or so and get access to the entire "DM Toolkit (tm)" which includes most monster tokens, basic maps, etc. Enough to run a game. You can also buy complete modules and special tokens for extra money, if you like, but the sub should be all you need to feel fully empowered as a dm. That means ways to add homebrew as well.
Most importantly, there should be a way for players to contribute, as part of their own subscriptions. So if each of your 5 players puts in an extra $3 a month, you get the toolkit for free.
There might be a way to buy the MM for lifetime access, but then you'd need to buy each new MM package (and other sets like a book of magic items) as they come out. The subscription would just be all the non-exclusive monsters (exclusives mostly being named npcs from adventures)