Not practical for the average joe to build one. If they're truly made from scratch, soldering the miniB side is a pain because the 4 pins are so tiny.
It is tricky, but as long as you take your time, it's not too hard. You will probably ruin your first USB Mini B connector doing it if you're not careful, but as long as you keep your temps in check and tin the wires you should be fine.
Mini USB Kits:12 GBP
Unterminated Kits: 11 GBP
Filco Kits: 13GBP
Ergo Dox kits: 22GBP
Any good?
Not trying to threadcrap, but those prices seem kind of high to me unless the kits come with sleeving, shipping is included, or your parts costs are a much higher than the parts I source. $12GBP comes out to almost $19 USD and that's not including shipping. I was talking with Badwrench when I first started making my own cables about doing a DIY kit since he expressed interest, and I was planning on charging around $10-12 USD range shipped for a 6' cable kit that comes with instructions, a 6' length of cable, cut heatshrink for the ends of the sleeving, USB Mini B connector and housing, and a USB A connector and housing.