depends greatly on what boiler you want as you could have a heatline for �450 or a vaiilant or worcester for �800 - �1200. This is obviously going to reflect on the price of the final system and �2400 to �3000 would be a good price.
Be very careful in your choice of boiler, your chosen installer may well specify one but that could be because hes getting a good discount or incentive from the plumbers merchant. Vaiilants are the best boilers on the market, worcesters a close second.
you also need to consider your hot water, most people switching to a combi for the first time are very disapointed with the temp and delivery rate of the hot water. to get a good flow of hot water you need the most powerful boiler you can afford i.e. off the top of my head the vaiilant ecotec combis come in 24, 31 and 35 kilowatt, 31 is the most popular choice and should do the job you need.
note that the central heating is irrelevant in your choice of boiler power as any combi can power the heating as the water is circulating thru the boiler constantly whereas with hot water the cold water passes thru the boiler on its way to your tap so needs more heat.
where do you live?