It’s not cheap to manage athritis, that is for sure! I would try Smartflex Senior and then a topical such as Surpass as well. Otherwise, you can try a basic joint supplement such as Cosequin, and add in BL Pellets or something to add herbal anti-inflammatories- but just be careful since she is already on Previcox. There are a bunch of combos you can make up yourself in Smartpaks so it may take some experimenting to see what works and is affordable.
If oral supplements fail to provide adequate relief, it may be time to add an injectable to the treatment plan such as Legend, Adequan, Pent Aussie, Pentosan or Polyglycan. Each one is a bit different, some are IM and some are IV so it may be worthwhile to ask your vet if those are an option.
Has the mare ever had her hocks injected? It may be time to consider that possibility as well, much as we don’t want to. I am facing the same myself and it’s not easy on the pocketbook.