I just paid £400 (for an Isuzu Trooper) doing what I would call a middle job - it took most of a day, they dropped off the wheels and worked into the arches etc. - they also pumped sealant into the sills etc. The car was clean underneath before they started, although they treated a couple of patches of surface rust before waxoyling. So it wasn't an "everything off" job, but neither was it a superficial coat over the easy bits.
I reckon it was reasonable value for what I paid - given it was a good 6-7 hours work, to do a full strip and seal job would probably be at least three times as much given the work it would involve.