Approximately �600 - �800 depending on who you buy it from, and how good the parents are. Check out http://www.champdogs.co.uk click on Guided Search, then Utility Group, then Boston Terriers for details of breeders.
(They are not in the Terrier Group!).
Have made a few enquiries, and it would appear I was a bit conservative in my last estimate! They can fetch around �,1000 - �1,500 if bought from one of the more successful kennels.