Central Heating cost

I am looking to help out a close friend by paying for them to have central heating installed and I am wondering what the cost is likely to be, I can't ask for qoutes without spoiling the surprise, well they would need to view the property first! So I dont expect an exact figure, just a guide. It is a one bedroom town house, entrance hall and downstairs bathroom, stairs up to lounge/diner/kitchen, double bedroom. Boiler would be located on an outside wall, a few feet above an existing gas fire (to be removed) so gas supply is not a problem. Heated towel rail radiator in bathroom, rad stats and a combi boiler. Any ideas??? The property is in Grimsby, so no London prices please ;-) Thanks Johno