As for a simplified ADSB technical overview visit
http://www.flyadsb.com/techoverview/techoverview.htm (i think its an faa site). Basicly ADSB transmissions happen around once a second and there doesnt seem to be any extra cost attached.
I know over here in germany we are required to equip all commercial airplanes with Mode-S transponders in the "Advanced Surveillance" mode which transmits a lot of data, for example position (sic!), track, heading, altitude, rates, selected altitude (you can then see what we preselect and correct us), callsign etc. With that part of Mode-S you can basicly get airplane derived position as well and use that instead of radar calculated which is similar to how ADSB works.