What sort of price should a new boiler be?

As you have given no details at all of your existing system I will ume you have a normal domestic boiler installation with an airing cupboard with a cylinder and the suggestion is a combi then the price is not unreasonable but there are so many variables involved in this kind of job the price could easily vary from 1800 to over 4000 or thereabout especially if you use a well known national company.