MorganS:
you don't care if the transistor costs $1 more, it's easier to design with the switch on the positive lead from the battery.
You can get logic level power MOSFETs at less than that. I paid RMB 0.18 for the IRLML2502TRPBF n-channel, and RMB 0.20 for the IRLML6401 p-channel. Switches 3-4A loads just fine with 3.3V gate. That's a price difference of RMB 0.02 (about USD 0.0033). Minor disadvantage: SOT23 packages.