Is there a prebuilt PCB that does the equivalent of a wall wart for a similar price?
Shopping questions are generally discouraged but I'll answer by saying your search term is "AC DC converter".  Digikey offered a bunch of subgroups in the answer, and I selected "Power Supplies, Board Mount".

http://www.digikey.com/product-detail/en/VOF-6-5/102-2212-ND/2441885 is $14, down to $11 in quant.

http://www.digikey.com/product-detail/en/BP5063-5/BP5063-5-ND/658569 is $8.

To add a bit more value than just the shopping aspect:
Be prepared to spend a good deal more on AC inlets, fuse drawers, and such.  Be sure to spec out enough output power on the power supply you choose.  Also, the addition of an internal power supply can really change the flavor of a prototype.  You'll need a bigger box, and make sure you think about this in 3 dimensions, not just two, our your box may not be tall enough.  Murphy's Law says the power supply will get in the way of every connector.