To the best of my knowledge unless you are with a fire sprinkler company you cannot touch a backflow device on a fire line. I can only test irrigation and commercial (medical, foodservice, etc) backflow devices.
I'm sure it depends on what region you are in. I keep hearing around here 100.00 a inch is the going rate. It makes all the paperwork worth it.
I am one of a few few (under 10) that test Backflows around here. And we have 100-150k people within 20-30 miles. Supply and demand? I guess if there were more testers the price would go down?
High end commercial, multi-family and residential lawn care, irrigation (LI and BPAT), and landscaping.
Mikegyver Pics Thread