I take them to a company near me in Ipswich, Suffolk. They do all my saw blades as well.
I cant remember at the moment exactly how much I paid for the last lot of worktop router cutters I had done, as I had a few saw blades sharpened and it was all on the same invoice.
I THINK, it was about £8 a cutter.
I have never noticed any discernible difference in the joints after using a cutter that has been sharpened once or twice. Obviously, the overall diameter is going to change every time they get sharpened, but as someone has already said, it is so small a difference that it has never been an issue for me and I have been doing the job for more than 32 years now.
About 18 months ago, I bought a job lot of 10 worktop cutters from Axminster. They were a very good price (about £65 IIRC), but they lost their sharpness very quickly. I have had a few of them re-sharpened, but they just don't seem to cut as smooth or as balanced as my Trend or Titman cutters that I use the majority of the time.
Tim.