linear bearing prices and numbering

Hi guys,

I'm going to be constructing my own hobby 3D printer and I figured I'd go ahead and use linear bearings on all the rods. I'm confused about the prices and numbering tho. A LM8UU 8mm bearings can be had on ebay for about $1 each and LM12UUs for less than $3, but 1/2 in. bearings are $18. Is it that they're made overseas so they tend to favor the metric sizes? Also, is there somewhere I can find how the numbering works. Obviously LM8UU and LM12UU are 8 and 12mm, but I haven't found anything on Imperial or SAE sizes.

Thanks for any guidance,

James