Roughly how much does it cost to replace your boiler?

In my flat I have a really old boiler, not sure what you would call it, but you need to put the water heater on on and give it 30 - 60 mins to heat up (depending on how much water you need). I'd like to replace it with a new combi boiler but was wondering roughly how much this costs? I know you can get free quotes from places but I need a rough idea before going to the hle of taking days off work to get people round to do quotes for me. Any help would be appreciated! Thanks!!