I don't think money is the biggest issue tbh. It's more about the creative stuff. Larian also probably wants to establish a long-term relationship if possible. I mean, Kirill was no "man on hire", he was a part of the Larian team.
What I found special about Kirill (beyond his pure composing skills) was that his soundtracks for Divinity (especially for DOS) covered an extremely huge range of different sounds, instruments and atmospheres. In most games there is one specific sound and a given set of instruments that are use for everything, but Kirill managed to make a ton of partially very different tracks and still put them together to one consistent soundtrack. I can't think of many game soundtracks that could be compared to that...
Man, I really just want Kirill back. The best die always way too young...
