I built my Freetrack setup for around £7 ($11) and that was only for the power adapter because I didn't like the idea of always changing batteries. It's a damn sight cheaper than TIR and works just as good. All you need is the Freetrack Software, a compatible webcam, an old baseball cap a bit of wire and three IR LEDs like SFH485P or equivalent. I got mine from Farnell. I used some wire from an old IDE ribbon cable.

Because I used a power adapter switchable between 3, 4.5, 6, 7.5, 9 and 12 volts I didn't need resistors. The voltage worked out at 4.5 so I tried that but the LED's got a little hot to the touch so I dropped it down to 3v and it's worked cool and flawlessly ever since. The stated voltages on these cheapy power adapters read a little lower than the actual output, so be aware of that.

Build one, you won't be sorry. Cheap, easy to construct, functional and 6dof. Worth spending a little time setting it up accurately in the software.