Think you mean a gnav i.e. like the 29er, upsidedown kicker (vang)?
Boom:
As a guide 300mm wide uni is £12 pe rm (inc resin)
A boom uses 2x its length in uni and then about £30 in weave/twill (inner and outer layer).
Including consumables and faff then it is about £60-75 all in (the numbers above add up to less).
Strut
A strut for a vang would be about 1x 300mm strip in total, but others will know better.
Others
Then you need to make a slider and add a goose neck on your mast (or silder on the mast and pivot on the boom).
Not sure how your rig is set, but there probably is also a need to move your lowers to the vang point to avoid the lower mast making a nasty shape. I'm sure others on the forum will chip in more useful stuff, but a boom and parts is a good sticky weekend project...
