I paid 35 bucks for a pair of inline cyclocross levers on ebay. It's carbon fiber bling, I think you can get alloy ones for 25. The other thing you most probably gonna end up needing is a longer cable, so buy one of the tandem cables in advance. Get a pack of brake housing as well.

Then on my right handlebar (the rear brake) I ran a cable from the main lever up the right airbar and through the inline lever, loop it and ran it back down the aerobar. If you have the tools, it should not cost you more then the cost of the components plus the time. I just paid about 90 bucks to the mechanic to get it done, simply because I do not have the time (and the skill, probably) to do it myself.

The other way to do it (i thought about it too) is to buy the same cyclocross levers, but instead of looping the cable use a brake cable doubler (produced by Problem Solvers, 40 bucks or so).