How much to service a reg?

[knotical]

Parts consisted of o-rings, washer filter, diaphragm, h/p seat, l/p plates, back-up ring, filter, circlip, and ty-wraps. They gave me the used parts back, and they don't look like they'd be worth $100!

If they did a first and two seconds, that is three service kits. If by l/p plates, you mean low pressure seats, then all the parts sound like they were in the kits and if so, no special parts were needed.