Well, I build my own 5 v box mods... but I know enough to solder, have an iron, solder, etc. If you're into modding, building your own is fun. See the modder's forum. If you already have an iron and solder, or can borrow some with a few basic tools, modding is the least expensive.
Modders Forum
Box, switch, regulator and a 510 connector is about $10.00 USD ish. 2 Batteries and charger about another $20.00 ish. Your prices may vary, that's a real rough ball-park figure. Total $ 30-35 for a "kit" and your own labor. Of course, you'll want a 2nd pair of batteries, and an atty/carto or two.
If you're not into modding, that kazoo-mod in Skyycamel's post #2 looks to be a fair deal. They have a bundle with charger, box, and batteries. I haven't tried em tho. They say they have another mod "coming soon" that may be a vv or multi-v mod too (unsure right now... very little info on site). Looks to be interesting, and fairly priced IMO.