Here is my recent experience with ponchos at a soaking wet soccer tournament. The $1 poncho will keep you as dry as the $15 Disney poncho. Realize that neither are going to keep you completely dry. You will still get wet, but not soaking wet. Depending on your height, they will go down to maybe your knees (I am 6'). Durability will be an issue with the $1 poncho. It is not going to last 10 days, but it will last a good 4-5 uses. It can be compacted down to fit into a pocket. The big Disney ponchos do not fold up as easily and will take up some space in bags.
The way I see it, I can pack 2 $1 ponchos in a backpack for each person that takes up very little space and almost no weight. I can buy enough for an entire trip for the price of 1 Disney poncho. Since I make a Wally World run anyway, I can get Disney ponchos there for $3 if I want the bulkier style (same thickness of Disney's $15 poncho). Wally World is where I got the $1 ponchos and they work as good as the expensive ones. Plus, you don't care about throwing it away at the end of the trip.