You can get a 10 pack of Ponchos on Amazon for under $12.

I don't know if I would ever tote along more expensive, reusable poncho just due to the space it would take up, plus having to lug around what is basically a big wet blanket after a storm.

edit. Be sure to recycle your plastic ponchos. Or at least put it in the recycling bin so it does not go "directly" to a landfill.