I am in the midst of renewing my passport, and will also need to get an International Drivers Permit before traveling in May. Both require recent passport photos. I went to AAA to look into this. At AAA 2 photos would be $15, 4 would be $30. The AAA staff suggested I go to Costco where I could get more photos for less cost. I went with a friend, since I am not a Costco member, and got 4 photos for $4.99 in a few minutes. Nice saving there.
If you have a CVS pharmacy, they do passport photos as well. Call before you go, but I had mine done there and they provided me with 4 pictures for about 5 dollars. Had them ready in 10 minutes.
Another option is to take a picture yourself (with a decent camera - a DSLR probably), size it to the correct standards (2x2) and send it to Walgreens to be printed. 20 cents per print, or something like that - and you can fit 2 with space around them on a 4x6, more if you really want to.
It's been my experience that passport photos don't just have to conform to a particular size of print. The face size must be within certain parameters. Taking your own pictures might not conform to this standard, and the application could be sent back for new photos.
When I worked at a camera shop here in Canada, the passport photo regs were very stringent...no glare/shiny spots, no shadows, white/light grey background, head size had to be between certain size, no smiling, no teeth showing, no hair blocking the face, and the back of one of the photos had to be stamped with the name of the store where the photo was done and the date. Maybe the US isn't quite so strict...it was amazing sometimes what some of the photos would be rejected for...heaven help you if you had oily skin that gave a glare! And taking pictures of babies was so not fun - you try propping up an infant of like 3 mos old and try and get their photo without any hands in the picture, or them crying, or trying to keep them awake so their eyes would be open...I don't miss that!
I should have specified - you need your face to fit certain parameters, as Nancy said, but the photos are also printed as 2x2. Here are the requirements. The face size must be between 1 and 1 3/8 inches, plain white or off white background, etc... It is doable to take them at home if you follow the sizing and other instructions.