Instructables has a great DIY solution for an iPhone Macro Lens that only costs $15 to make.
The instructions were posted by appsman:
When I couldn't find a macro lens for the iPhone 4 that had decent quality for less than $60, I designed a slide-on lens holder that can be printed out at Shapeways: http://www.shapeways.com/model/328790/
It takes a 12mm glass lens that can be bought from Surplus Shed (http://www.surplusshed.com/) for $4 - part number L4471. That was the cheapest place I could find for glass lenses.
In the left side of the web page enter the part number to put the item in your cart. Or search for the following lens. PCX, size 12m, focal 55. It's there for 4 bucks.