Depends if its still all connected (gas pipes) . and if the original installation had adequate diameter pipes and you didn’t add other stuff like gas bobs or boiler after so the original pipes are no longer adequate.
If you have to pull up the floor then costs will depend on the type of floor.. i.e. if it runs through the kitchen and the kitchen is tiled…
We had to re-do all the gas pipes except to the fire as that was on the end as they were inadequate to feed the hob + boiler + fire but we had the floor up anyway and the cost was less than I expected… (I can’t remember exactly but it was £300 or so?)
On the other hand we couldn’t touch the pipes for the actual fire as it’s not possible due to it being flueless and would need a permanent hole in the wall (one that can’t be closed) to meet regs….
Another thing to beware is if there is a back boiler still in a chimney breast… often gas fires are taken out but the boiler left in as it can mean knocking out a lot of the chimney to get it out…