I agree - have a look on ebay as you can often get good deals there.
Derby House often have offers on rugs, so it might be worth having a look there, also try equestrianclearance.com as you can get quite good bargains on there, although they don't always have the size I need (for me anyway!!)
JMHO, but don't go all out and buy heavyweight if you are unsure. You can always layer underneath if it gets chilly. Also, a lot of people just use turnouts instead - me included.