I just got an estimate from Airpets, a company based near Heathrow, for about £1599 to take my rather large greyhound to Washington, DC (I wrote about this in the wrong place in the repatriation board, without realising there was a special board for pet issues -- sorry!)
That included £1200 to fly him BA cargo. (Virgin quoted me £567, the bulk rate for 100 kg of cargo, but Virgin cargo doesn't deal with agents apparently.)
The rest of the Airpets quote was for a night's boarding (supposedly obligatory because the airlines prefer to take animals only on the very early morning flights so as not to hit U.S. customs at closing time, so they have to be sure the animals can leave the kennels bright and early), the veterinary fitness-to-fly certificate, a custom kennel, transport from their kennels to the cargo terminal, roughly £90-£100 for each item I think.
If you are looking at having to quarantine your dog, I think (check this with Defra) you can reduce the amount of time he spends there by getting his rabies shot and blood test for rabies immunity done right away. You can import him to the UK six months after a successful blood test. So if you get the test results next week and leave two weeks after that, he would have to spend five months and two and a half weeks in quarantine rather than a full six months. Hope that makes sense.