The next step down are firms that'll give you a platform including hardware and software for the "infrastructure" bits: often called an "algo container", but lots of brokers have an API you can use to avoid needing to write feed handlers, etc. eg. Pico.
Then there are heaps of providers who'll rent you servers, connectivity, rackspace, etc. You do all the software. Lucera is a trading-oriented "cloud" provider. OptionsIT or Fixnetics are infrastructure providers. Or you just go straight to the data centers -- any decent finance-oriented datacenter will have a POP for most of the venues, and you can just cross-connect.
No relationship with any of the firms named -- just examples off the top of my head.
80% of profits seems a bit rich, though. If they're supplying top spec colo servers, a super fast network to connect them, and people to stare at it, I'd say something a bit lower (but still pretty chunky) would make more sense.
I suppose the key for them is they have a bunch of existing infrastructure that pays for itself, so any extra cream is good. Also they have capital sitting around, so why not?
The strategies have to be more than unusually profitable, though. I don't think I've seen a down day in the 3 months I've been sitting here, so your average trend following sharpe ~1.5 or so ain't gonna impress people.
The big issue isnt actually infa it's capital. If you want to trade big you need a million bucks or something on hold with the exchange.
How do you borrow a mil to park at an exchange? The guys mentioned above.