The Open Academic Robot Kit

This is perhaps the cheapest option, with 5V regulators worth less than a dollar and a pre-made Battery Eliminator Circuit (BEC) for model aircraft worth a few dollars, such as http://hobbyking.com/hobbyking/store/__54275__HobbyKing_8482_Micro_UBEC_3A_5v.html or https://www.adafruit.com/product/1385 . We highly recommend going with a switching regulator (such as in most BEC circuits) to reduce heat and improve efficiency. The pre-made BECs are a good idea if you want to minimise effort; they're switching regulators (so fairly efficient) and already wired up with power smoothing components and flyleads to simply solder into your wiring harness.

This option does take a bit more effort (at some point you're soldering things together). But it does mean you only have one battery to worry about and is a lot cheaper.

If you do go down this route, you will need to make sure that the supply to the regulator isn't going to get interrupted suddenly. We highly recommend wiring the battery in parallel with an external power jack, with diodes to ensure that you don't get “back charging” when both are plugged in at the same time. That way, to change batteries simply plug in the external power before unplugging the battery. Again, this will allow you to keep the Pi running indefinitely, albeit with a bit more messing around.