Other big and cheap poncho/tarps?

Helium Hiking Equipment has one. James is a real good guy and will work with you if you want to change or add on to anything he offers if his specs don't match what you have in mind. I've placed a couple orders with him before, definitely recommended.

http://www.heliumhikingequipment.com/store/p37/Poncho%2FTarp.html

Edited to add link…also everything I have gotten from him have actually weighed in under what he states. Not saying that will always happen, but he definitely keeps weight low and has an honest assessment with what he specs online.

Luke's Ultralite sells one for $70 and MLD's simple silnylon poncho is $105.

I've never done business with Luke's so I can't speak to the quality of their work or their poncho.

From what I've seen of Ron's work, I'd feel comfortable using one of his ponchos as a tarp.

Edit to add: both are obviously more expensive that what you listed.

Luke's Ultralite's quality is very good. I received a silinylon rain jacket from him this week and I'm very happy with it.

I have Luke's poncho in sil and I am very happy with it. Best price and weight on the market currently imo and I was impressed with Luke's work.

With that said it is not a poncho tarp but rather just a poncho. It had a couple d rings so you can run a chord through to use as a belt when it is windy but there are no tie outs.