To a Vintage guy who wants a Simplex, maybe a couple hundred.
Check on vintagekarts.com for a better idea.
Forum link at left of page.
Having said that,if the tank is straight it by itself is worth a couple hundred.
The steering wheel & pedals also have value.
What the frame is worth depends on how original it is.
Original and straight without any additional welding done to it could be worth a few hundred.
Like many Vintage Karts, it's probably worth more parted out than complete.
On the other hand, if someone really needs the tank they may be more inclined to buy the whole kart.
Search eBay "Vintage go karts Simplex" for an idea of what's being asked for parts.
Registering and posting on the Vintage Kart site is worth your time, expect a day or two before you hear back from them.