You get what you pay for so I would not get a cheap one. It will be cheaper in the end to buy a good quality one
I use Stride Plus although about £50 to start with you reduce the amount you use as maintenance. I also use Flex from Nupafeed this is Greenlipped Mussel http://www.nupafeed.co.uk/dogs.htm?ac=BHVYF-K
My lurcher (Beddie x Whip) had a cruciate lig done 2 years ago this coming Jan. Since the op she has been on Mobile Bones powder (Pooch & Mutt - family that do Blue Chip stuff) £9.99 per pouch that lasts her (16kg approx) around a month.http://www.poochandmutt.com/products/mobilebones/ - (Amazon do free P&P)!
Also has cod liver oil (get human stuff from Tesco) about £2 per bottle. So far touchwood not had any problems since.