It was about 80p a kilo but I think it's a lot less now. Apparently, Potters has a nine week waiting list for horses to be slaughtered. . I can't remember where i read that and I can't find a source for it (mostly because I can only find references to Harry Potter on google. Grr!).
You'll probably have to phone them and ask what the price is now.
I should add that they only pay for the actual meat - bones and intestines etc are not included in the price. The actual price you get for your horse is based on about 50% of it's bodyweight.. I think. I've never had any of mine slaughtered. Been tempted a few times though!!!