We never purchase rain ponchos in the parks. They are between $8 -$9 each. Before we go to Disney, I always go to WalMart and purchase the 88cent emergency ponchos to take with us. We have only needed them once in the seven times that we've been there. The Disney ones are only slightly better quality.
Off site-2003, CB 2007, GF 2008, WL 2009, CS2010, Dis Dream and POR2011, 5 months until YC
We have considered the disney rain ponchos to be a souvenir. They lasted us several trips back. They are nice and thick, and don't cling to the body like saran wrap, like the dollar store ones did the one trip we tried them.
But now hubby and I bought serious rain ponchos from RickSteves.com, and they are fabulous. DS has a Universal Orlando one that he LOVES and is going to wear until it's shreds.
For us, the cost of constantly replenishing the poncho supply, and the non-financial cost of disposing of thin plastic all the time, was too much.
I got mine at the Gag Factory in Toontown, it was around $7 and it's great! It's heavier than the cheapo ones at WM and it's lasted for about 4 years now, I don't use it often but I always take it to DL (it lives in my suitcase).