Yes, it IS split between base game spins and features, in my earlier example the game pays 3/4 (72%) of its RTP in the base game, 1/4 (24%) in feature so just over 96% overall RTP.
All that happens for bonus buys is that you continually play the feature so that 3/4 of base RTP is now allocated to the bonus round too (bought at significant cost of say 100x stake) which is effectively you 'buying' the 3/4 of RTP for base game wins you would have had (on average) and then playing the feature which (on average long term) incorporates this and will pay you 96x bet for each of your 100x purchases. The same as the base game + normal feature would pay you 96% overall in the long term.
If you check game rules on BB slots they will have the RTP listed something like this:
Overall Game = 96.00%
Feature Buy for 100x bet RTP = 96.03%
Super Feature Buy Option for 200x bet RTP = 96.04% (if applicable)
So in other words the rules will list normal RTP plus the RTP figures as if the game was only played on each of the various BB options.
P.S. I just read your last post where you make the incorrect observation that you 'would not be getting the base game 65% if using Bonus Buys!
You do! What's happening is the base game RTP and feature RTP are added together when buying features. like this:
NORMAL:
Base game feature frequency on average = 400 spins.
Feature allocation/proportion of overall RTP = 25%
BONUS BUYS:
Feature Buy cost = 100x spin stake
Using Feature Buys you are sacrificing the 3/4 of the overall RTP the base game would normally pay in those 400 spins.
So Feature is occurring in reality 4 times more than it would in 'normal' play (1/400 spins.)
So your Feature Buy RTP is 4 times more than the 25% of overall game RTP it's allocated in normal play.
Therefore using bonus buys, 100% of the game's stated RTP is allocated to the feature.
Perhaps
@trancemonkey would explain it more simply than I have.