I had my mare done 4 years ago and it was about £45 and £12 per year registration after that. I wouldn't say it hurt. My mare is grey so she had it done on her shoulder as the irons have to be left on for longer to make a bald mark. She didn't move at all while it was being done.
I had a few horses done in 2011 - I had them done on shoulder or flank area and in both cases the greedy ponies didn't even notice it was happening as they were too busy inhaling readygrass.
I had swift done last year, under the saddle, he was a bit shocked about someone sticking cold ions on his back but didn't seem bothered other than that! it cost me £70 ish but i paid £20 extra to have a personalised mark so £50 ish for a plain one plus the registration fee, can't remember how much it was now i think i opted to pay up front for 5 years as it was better value that way
I suspect that it's more uncomfortable rather than painful but the area then scabs over and is tender for a few days. I think I was recommended not to ride for about a week as my lad was freezemarked in the saddle area. Can't remember how much it was now sorry as it was a few years ago - you can get a personalised freezemark but they are more expensive.
I had farmkey and cant remember costs(dont remember it being extortinate) but they were really good. I had a bag of pony nuts and went round and round at head end and kept feeding himthe lady stood behind me at his shoulder freezemarking him. She just got on with it. Its the same as having a wart removed(if you ever have). It hurts I am guessing later on, not whilst its being done. It does scab and blister but healed really quick.
Farmkey did mine twice as he is cremello/palamino but I would definitely not bother with a light skin/colour horse and it only shows up with his summer coat. I do think a brilliant visual deterrent though.