Something people miss is that buying the feature often changes the published return, sometimes up by half a percent, sometimes down. Providers list both figures in the info screen, though you have to scroll past the paytable to find them. A few comparisons of base game versus purchased feature RTP are laid out on https://thebonusbuyslots.com, which saves opening ten different help files. The gap is small in practice, but it exists and is worth checking before you commit.