Passport photos are oddly expensive for what they are - 2x2 inch photos. Meanwhile, a 4x6 inch photo is usually 33 cents or less. Therefore, if you're handy with a photo editing program, you can tile 6 2x2" photos into a standard 4x6" print.
1. Take photo with white background
2. Size into square (of any size, but higher res is better) with proper passport requirements. Check the TSA website for proper size requirements.
3. Duplicate that square and tile it into 3 squares by 2 squares. This naturally gives you perfect dimensions for a 4x6 photo.
4. Bring to local photo store/pharmacy on CD, USB drive, SD card, etc. It costs like 33 cents. Many places have instant prints nowadays.
5. Cut photos out. You have 6 passport photos for 33 cents. Repeat as necessary.
For the price of a standard set of 5 or so passport photos, you can get 60 of them. Then stop worrying about how much you'll need to print.
I did this for my actual passport. It also guarantees you don't look stupid in the photo since you take it yourself. But make sure you know your way around the photo editing program.