I actually went the other day but didn't go riding.
Getting there is easy, take the 59X from Mei foo, the 962 from central or the west rail. get off when you see the tune mun ferry pier. from there it is a 5 - 10 minute walk or take the light rail to butterfly.
I think it was around 300 per half hour for riding but there is an 80Kg weight cap - not sure why as the horses I saw were all regular quarter horses (with one very cute miniature pony).
Good luck and have fun! i wold be interested in hearing form anyone who has actually gone riding there - what kinds of rides, where do you go, what sort of pace etc. Please post again if you end up going.