$100 for the gt2-a frame (gasbike.net)
$180 for the forks (monark springer)
$160 for the 66cc slant head motor kit
$xxx for whatever donor bike you want to transfer parts over to.
I built one. Cost me right at $500 (ish). I love the bike! Solid construction, light, and very smooth. I used a 48cc, so your experience may vary. I've never heard of that company, so I have no idea if they do good work or not.
Looks like a good bike though.