Someone posted recently that they had to pay the full adult price, but I thought it was 85% of the adult price normally - it's certainly not cheap.
Would I recommend it? Depends on how old the infant is. if the infant is quite small, then I would request a skycot (which is free), and use that - then baby can lie flat and sleep.
For an older baby (over 1, under 2) then I would consider the carseat, but BA and Virgin both offer an infant seat which is cheaper than paying for a seat on the plane, so it would be worth pricing both options. I think for a wriggly toddler, then car seat would really come into it's own, as the child would be familiar with car seats and understand that it can't get out and walk around.
we used a car seat for our 3 year old and imho it was a nightmare - she had flown before without one, and could remember it, so she knew a carseat wasn't a requirement and she totally objected to being in it. I had wanted to use it, because I thought it would be more comfy to sleep in (since it reclined), but she wasn't having any of it, and there was noway we could remove the carseat once we had taken off, so she was stuck with it - it was a bit of a nightmare.
Consequently, I have quite mixed views of taking one, especially considering the expense - but I do think for toddlers they could be very useful. Also if your baby is 2, then you have to buy a seat anyway - so no additional expense makes it seem like a good option.
Bev