Is your horse okay with IM injections?
Pentosan injections are about the same cost as an oral joint supplement. Just call your vet for a script from wedgewood pharmacy. I used supplements for years and never really saw any difference. The problem with supplements is they aren't always absorbed well from the GI tract, and unless you want to pay $1/day or more, it is hard to find a supplement with enough active ingredient to make a difference.