During normal operations, the "fee" is set to be a small fraction of .01 USD. This is for ALL transactions, regardless of the units being exchanged through the system.
This is a protection measure for the ledger against spam attacks. If there was an attempt to overload the consensus system, the fee would increase in increments. It would still be a very small amount for a normal user and would be a financial hindrance to a spam attack.
There is no monetary compensation for being a part of the consensus. The base fees and load fees are destroyed. Ripples are not mined, it was all created at once. People will likely be running their own servers for fast, secure access to consensus, not coin rewards.
A basic and technical overview of transaction fees and how they escalate to prevent spam
How consensus works.
"Finding consensus by trusting validators not to collude is a simple and strong way to establish the valid ledger. Because, the network is not relying on proof work, this consensus process is fast and allows the Ripple network to validate ledgers in seconds."
An excellent visual explanation of consensus. Also shows how transactions can be verified within seconds as opposed to the minutes required for many other crypto-currencies.