On a high level
SSP's have 2 business models
-- Ad-serving and rule engine
Which lets publishers create direct campaigns on their own inventory, get adtags from third part DSPs and ad-networks. For this exchanges typically charge anywhere between 0.015 to 0.005 CPM to the publisher for using their tech and infra (DFP for small business is free, for publishers doing under 90Mn requests a month). The rule engine is to maximize monetization, where in you can set rules like waterfall: provide the inventory to the higest monetizer, once the fill or eCPM drops, move your inventory to the next highest moentizer. You could also set flat rules, like 30% to this demand partner, and 20% to the other, and 50% to the exchange, etc...
-- RTB
The remnant inventory that the publisher is not able to sell directly, is diverted to the exchange, here SSPs charge 10% (could vary from exchange to exchange) flat of the money they earn for you.

DSPs, charge a flat % of the media spend of the advertiser. So if I an adv use a DSP to buy media, and I spend USD100,000 through a DSP, I owe the DSP 15,000 (if its 15%), DSPs vary the percentages based on customer loyalty, importance and scale

So to summarize:
DSP charges the adv 15% to use their infra and tech
SSP charges the Publisher 10% to use their infra and tech
so the cost chain for RTB is about 25%