One sample doesn't really establish a "market rate" yet, so I don't worry about the seller not being motivated by financial profit motive. Sometimes the value provided to the seller goes beyond the financial payment. 18F gets a good deal here because of the intangibles they have to offer.
If they put out more projects than the amount of work people are willing to do at the "$1 volunteer" or "$1 build my portfolio" rate, then the price should go up. How deep is this pool? I guess they will find out.
The big challenge, I think, will be if bad actors start coming in and making crazy bids and then doing sloppy work. Time will tell.
I'm excited to see how this develops.