I paid £41 for a bag of Baileys Outshine. Depends how you're feeding it and what the purpose is. I was looking to feed it for weight gain and condition and the bag would have only lasted me about 2 weeks with my 17.2hh lad. Feeding for coat shine you only need about 1 cup per day though so it'll last a whole lot longer...
I find Soya Oil is good, I have been feeding it all winter, now his summer coat has come through really shiney. Also he didnt get any of that horrible dandruff most clipped horses get in the winter when they are rugged.
I put vegetable oil in my boys grub, always get comments on his coat, and he gets wonderful dapples. Also Robinsons do this fantastic brush which is half and half, does a lovely job of removing dust (searched their web site and don't think they sell it any more). Oh and not ideal for everyone, but a vaccum. At my last yard, we had a henry vaccum which does a nice job on coats. But obviously not everyone has the kind of horse they can vaccum!