I've heard good things about rust check. it's like $100 a year for their service, and is definately worth it says most people.
I just got my car undercoated at my mechanic's shop since it was already in there for a variety of other things and it was about $85.
So i guess you could say you'll be looking at anything from $60 to $150 depending on where you go and what material you want them to use to undercoat the car. I chose the thick tar like substance as opposed to the thin oily like stuff. I've also seen a dry graphite like coating on some cars before...
'94 Supercharged E36 325is (150,000 original km's)
-Vortech V9 G-Trim @ ~10psi w/ BOV
-255LPH Walbro fuel pump
-TEIN SS Coilovers w/ Pillowball mounts
-Keskin KT1 17x8.5 & 17x10
-Remus full catback
-Authentic Rieger Infinity I body kit
-SMT6 EMS (Untuned)