I use them all!!
QK dies are priced according to the size and if they are doublekutz or singlekutz.
Usually 4x4 are around $16 for singles and $24 for doubles.
2x2 are around $8 for singles and $12 for doubles.
I buy most of my dies online and at Archivers (they get exclusives). Scrapyland.com and scrap-mart.com have the best prices, but not always the best selection.
Ebay is often cheaper yet...