Prices for replacement boilers can have a marked difference depend on on what size & what type of installation so firstly look for an online Prices Guide to help.
You can find that there are 2 or 3 main considerations, such as combi boilers, system boilers & condenser boilers, although condenser types are really a variation on system boilers.
The UK building regulations require closed systems to be of the condenser type, and this is a benefit to homeowners as it can raise the efficiency of them to over 95% thereby potentially saving hundreds of pound sper year on heating compare to those that do not have a condenser facility
you can see more at www.mylocalprices.co.uk/boiler-replacement