Signed UDFAs will get a little money in the form of a bonus. Their contracts, however, usually aren't guaranteed and can be terminated at any time. NFL teams have a pool of money they're allowed to allocate to UDFAs as signing bonuses (in recent years, I heard it was $75,000 total). In 2011, for example, the Seahawks gave the biggest chunk of their signing bonus allotment to Doug Baldwin ($17.5K), Jeron Johnson ($15K) and Pierre Allen ($10K).
Mini-camp invites are like pure tryouts. It's like going out for your high school team.