If you are after just pure MSM, Equine American do a 454g tub for £14.99 (approx 2 months). Global Herbs do 1kg for about £35 (about the same area re price as EA). These people on ebay do 1.1kg for £17.99 but postage is £10.75
I use NAF MSM and for the small tub its £15.99 for a 37 day (I think) supply and then its £40 for the large tub , dont know how long that one lasts as havent bought that size. Been using it for my mare who tore her DDFT and it really does seem to help.