Cost depends on what fittings you're using and how easy access is. If the floorboards above can be taken up fairly easily that won't be so bad (and no replastering/painting needed, they can just punch holes in the existing ceiling).
Decent quality fittings (eg Aurora M10 fixed led downlights) will cost about £25 each. Count on at least the same again to install if access is fairly straightforward.
Consider whether you really want downlights in a lounge. We've just had a load installed in our kitchen diner and it works well there, but downlights can never give a completely uniform light (there will always be some areas in shadow). In our lounge extension we went for spotlights aimed upwards at vaulted ceiling, combined with a tall lamp, which give a much nicer diffuse light for a living area.