Even the best open flue gas fire will only achieve around 85% efficiency, some open flame living flame fires "boast" 50% efficiency. with many a lot less. when you get to the basket type then you really are watching your money go up in smoke.
The current condensing boilers are far more efficient.
By way as an advert for Electricity, all electric fires are 100% efficient
( doesn't mean they are cheaper to run though
)