Generally you get what you pay for. I'm using the Newmarket joint supplement, so far it seems to be working for him, and has been proven to work. Many of the cheaper ones don't have high enough levels to do much good, and haven't been scientifically tested (according to my vet). Newmarket works out about 60p a day for a 500kg horse.
i use cosequin that i got from my vet tho i have now found a cheaper supplier on the internet. its expensive tho at 95 a tub but theres 220 scoops in a tub and my horse has about 1/2 scoop a day so lasts me for ages tried the superflex but wasnt as good cost me 35 and lasted me about 5 weeks so worked out more expensive any way