Like someone else said, you get what you pay for. If you are only getting it for extremely wet/cold weather, make sure that it has some sort of fill. Keeping them dry is important, but when temperatures drop and you blanket them, their hair flattens out so they can't trap heat. When they fluff up, they can keep themselves pretty warm, but it's tough for them to fluff when we blanket them :)
That being said, I blanket mine all winter cause they are a bunch of weenies! They all live exclusively outside (I'm in Montana), they are happier and enjoy the exercise. They do have outdoor shelters, but in the first two years I had them (4 of them) we went through 8 cheap blankets in two weeks(I got them from tough-1: I was drawn in by the price)... I bought rambos after that and they have had them for 5+ years... with minor fixes! They LAST! The $250+ (I've seen them upwards of $400) price tag is initially a shock, I was absolutely floored! BUT, you can find good used ones on ebay for ~70-120$; it is a great investment. Sometimes they need a little waterproofing or patch, but those are easy fixes if you don't mind whipping out the sewing machine or bribing a friend :) My guys are with 15+ horses and play HARD- and those blankets have stood up to their shenanigans with no problem! So, I would say invest in something used, but is a super high quality before buying something that falls apart with normal wear and tear. Hope this helps!