i'm £60 for a new set and a trim, £50 for a refit and trim.  I usually need a new set every second time. (farrier comes every six weeks).  I too have a thoroughbred and find that in dry weather her hooves tend to crack but this is due too her hooves being overhard.  I use keratex hoof moisturiser every day that the ground is dry during late spring/summer and it really helps.  If your horse's hooves are too soft though dont use this use keratex hoof hardener instead.  Ask you farrier if you arent sure.

i am thinking of having her back feet taken off, she is a thoroughbred so has very sensitive feet, i ride her 4 times a week  would this be a bad idea? also are there any good creams that would harden her feet?

Mine is £63 every 8weeks for a full set...