As a local SAFE KIDS USA coordinator, I volunteer at free events. Of course, advice is always free here at Car-seat.org. I also charge for checks I do on my own time (
http://www.seatcheck.net/services.html ). I mostly cater to expecting parents who can't wait for an appointment at their town's fitting station, if one is even available to them.
The sad reality is that many local departments have cut their programs entirely or restricted them to residents only, mostly due to costs. Aside from supplies and replacement seats, labor costs are a huge factor. The typical fire or police department employee is probably paid at least $30 an hour including benefits and overhead. I'd much rather see departments charge $10 for a checkup to recoup some of their costs than cut programs and put their effort elsewhere.
I could do checks for free, but I'd be overwhelmed with requests and I really don't want to be spending all my time doing them. As it is, I refer most people who contact me to free fitting stations and events. I charge what the market will accept and it limits the number of checks I do to a very low number. Someday when the baby is in preschool, perhaps I will advertise;-)