($1 assuming you already have some single bracket hardware. About $19 if need to get the single bracket.)
There are a number of ways you can get your flash heads together, especially if you are triggering them with radio triggers. I've used ball bungees to strap a couple together -- boy did that contraption make me nervous. Another way to go is one of those nifty double or triple brackets that you can buy, like this:
They start at about $60. I wanted to get one of these and then I started to think about how simple the mechanism could be. So off to the hardware store I went. I purchased a 3 inch long 1/4 in./20 headless bolt for 59 cents.
Now I already had several of the Impact Umbrella Brackets: http://www.bhphotovideo.com/c/product/298709-REG/Impact_3117_Umbrella_Bracket.html .
So, in conjunction with the CowboyStudio flash triggers that I've mentioned before: http://www.amazon.com/CowboyStudio-NPT-04-Speelights-extra-receiver/dp/B005IQRMN4/ref=sr_1_261?s=electronics&ie=UTF8&qid=1322752105&sr=1-261
I slid the new bolt in front of the brass stud at the top of the bracket (you may have to loosen the screw that tightens on this stud) -- then tighten it all up. I screwed on the CS triggers, and subsequently the flashes. And this is what I got...
I think that I will add some washers and nuts to prevent rotation on the two side triggers/flashes, but what you see here is pretty much my answer to the $60 and up tri flash bracket. For only 59 cents to $19. Now to add the ol' umbrella (or maybe a brolly box) and get shoot, shoot, shootin' at low power and virtually no recycle time!