Needs tweaking
I played another game like this a while back, but instead of firing the drill into the ground it was a little tank like thing that could drill sideways and down; it also had a booster for flying back to the surface. The challenge was to get as much stuff as you could before your fuel ran out because you needed to have some left to boost back out of your mine to refuel and upgrade. I thought that was a blast to play.
This is fun as well, and in it's own way it is very good! ^_^
Still, it has room for improvement, and I hope you take these suggestions to heart and make a sequel.
First, The directional arrows here are a bit awkward to use; Not at first, mind you, just after you get the cannon. the A S D keys would have been more comfortable to use while I used the mouse to actuate the cannon.
Second, using Enter is another awkward key choice, and had me having to press it to shoot the drill and then trying to quickly get my hand into position on the steering. Under your current control scheme, Up would have sufficed. If you take my first suggestion, then the obvious choice would be "W."
Third, while it does take a while to get upgrades, I file that under "learning curve." But others are right; you should show gotten upgrades, as well as prices for upgrades to come. (at least the next upgrade up). It would take some of the guess work out strategic shopping.
Fourth, While I understand the idea of speed here, the bullets you fire do not move fast enough to be an effective weapon while at speed. I found them more useful when I was coming to a stop to nab those last few gems, or fuel container. The problem is that often times I'd be right near a fuel tank but I had already come to a complete stop, and thus my ability to fire had gone away. I'd like to be able to fire for at least a few seconds after coming to a stop so I can at least clear the screen...actually, being able to "Pause" the drill (as an upgrade) would solve this problem perfectly! :-) (of course that would/could make the game far to easy, but then it could the last upgrade you get...or something.)
This is otherwise an excellent game and I hope you make a sequel!