Hello Alec & Welcome!
On an Indie title I think you can realistically expect to sell maybe a few hundred copies & (rarely) up to a few thousand. I’ve noticed that anything associated with the game directly does better than the OST on its own. For example, unless you have a pretty large fan base it’s hard to imagine that the majority of your sales will come from Bandcamp, iTunes, etc. On the other hand, bundles are a good idea & the OST up on Steam has performed the best for me.
I always recommend retaining as much of the rights & revenue to your work as possible, but if the company is willing to additionally compensate you for a % in the OST revenue then it’s worth considering. Especially if they’re open to the idea of OST + Game Bundle sales on multiple platforms. They’ll have some overhead for calculating your %, so they’d probably want to offset that cost & make a little on the side as well.