installation wise, presuming your flue was ok and the fire fits straight in then we charge 75, if the flue needed slight modifications then it would be about around 100-125 or if you wanted us to do the complete job including installing the hearth, surround and back piece then it would be around 200. if you wanted all the pipework concealed then it may be slightly more expensive. these prices are only a rough guide to what we charge as we are only a small independant firm and we tend to be on the cheaper side.
as a worse case scenario we could examine the flue and advise you that you need to get a builder in first to work on the flue, we had this recently and it cost a 1000 to get the fire installed in the end, this is on top of the 1000 the householder paid for there very flashy inset fire that was set half way up the wall. don't panic though, we install the vast majority of fires for less than 200.
so a quick re-cap, your getting rid of your old fire anyway, so call out a corgi gas installer and get the old fire disconnected and capped off and then get him to examine your flue and to advise you on what fire you can have.
if your in the birmingham area then let me know as thats where were based.
hope that helps, let me know if you need anymore advice
p.s. i should have said, with the real flame type gas fires you can get a decorative fuel effect type (dfe) or a living flame effect type (lfe) the dfe ones are designed just to look nice, they give out hardly any heat and cost a fortune to run, they also need additonal ventilation installed. the dfe's do look nice but go for a lfe.