I hate to be against the general consensus in this matter, but I feel for the vets that:
a) will be missing out on some pocket money here (a practice is a business after all and has very high costs) if everybody starts doing your FEC; vets love animals and their jobs, but they have to feed children and fuel cars too!
b) generally the vet that comes sampling your animals will know the farm/smallholding, the way things are done etc, and will almost always (exceptions allowed here, not all vets are saints) keep an eye out for other symptoms and general health check of the animals.
That is also why I don't really like the idea of sending samples out to a random company that does not know the farm. They will just give you raw data back and often interpreting is difficult. Even if they have a vet that signs off the results, they won't know the particular situation. This is where a 5-6 year degree comes handy!!!
And, talking to the vets always helps, even if you want to pester them about the pet cat whilst they are sampling the sheep. I feel that most practices will offer discounted/free FEC if enough/year are requested/planned; they'll be rewarded by a worm-free smallholding. However I think they'd always charge full price if only one FEC every now and then is asked.
Just my opinion (vet school was super-hard!!)