IF you got it for 9.99 that's a steal, there's an update coming May 8,
Portal 2 Perpetual Testing Initiative lands on Steam May 8 | VG247
and if you go to
Portal 2 Maps.net | Custom Portal 2 Maps there's quite a few single player and coop maps that will keep you busy. I've done pretty much all the coop maps and some of them are as good or better than some official Valve maps. Then there's single player ones, and they get even tougher.
For example:
[SP] The Ledge | Portal 2 Maps.net | Custom Portal 2 Maps