The biggest issues you're going to have to overcome is NOT a patent document, but successfully marketing your device!
I'm fairly certain you could get a utility patent, basically because it is unique, but you have to asked yourself WHY no one has done it before.... the reason why is because technology passed your device years ago.
Your device WOULD prove useful for those who need a DIY (do it yourself) homemade solution for vaporizing their stash, but there are self-contained devices already patented and on the market that do the EXACT same thing as your device, such as this:
Why spend the time and money getting a patent on something I can all but guarantee will NOT be marketable to the mass of smokers?
I would suggest you make a simple "instruction booklet" using your printer & a stapler, detailing how to make and use your device, then use portions of the $850 to place ads in "High Times" magazine, then sell the instruction booklet that way... maybe even offer devices you make to sale along with the booklet? Doing it this way will be MUCH cheaper than USPTO fees, and you'll more than likely make more money from sales through the ads than you'd EVER hope to make otherwise.
Please do not take offense because I'm not trying to be criticle; I'm just being straight-up with you.