There are a few such agencies out there: RedHat, Igalia, Credativ come to mind. There are some more listed on employers and freelancing sections of the FOSSjobs wiki:

https://github.com/fossjobs/fossjobs/wiki/resources#employme... https://github.com/fossjobs/fossjobs/wiki/resources#freelanc...

They seems to be insanely Cheap at $1999 for 160 hours of Dev work.

How do the make money?

They only pick up FOSS tasks for which they can find multiple customers.

Site's down / unresolvable. Sounds good - I'd work for them.