The Singapore books seem to be reasonably inexpensive no matter where you buy them. I've mostly bought mine from Sonlight, although ordering from them is kind of a pain. The Rainbow Resource catalog sells two versions of the "Singapore Method" and are comparably priced - maybe within 50 cents or so of Sonlight. You can also sometimes find them on e-bay, but it's hit-or-miss and with shipping you're not that much farther ahead.