I've worked on several h-games, and 5% might seem steep and depending you're right. It really depends on the market your making your games for, I can only speak from experience in the h-game scene so I'll toss out what I can. If you're paying a monthly fee, and your project gets delayed for whatever reason you can be negative very quickly. If it's per-sale after X amount, you're probably better off. Professional h-game developers that make 250k+ on each release and do 3-5 releases a year are going to find monthly better. On the other hand the small 2-5 person teams(fyi there's a lot of women in h-game development especially the small groups), 5% after 100k is a better deal. Since your average release will only get you $80-90k, successful small h-game can reach 250k+ or more though.
Now to the interesting part, what do the sites get in cuts? Places like dmm, dlsite, getchu, and so on take 25-55% on each sale, most use a variable pricing structure based on how much you sell your title for. Sell for more, they take less of a cut, sell for less they take a larger cut. The titles I've worked on usually see a 30-35% cut taken, which is about par with steam for example.