Nice idea! I like the upgrades, and the gameplay loop is good.
Criticisms/Suggestions:
The price of oxygen rises too quickly, cutting a single play session short.
It's unintuitive to have to stand on top of the hole you're digging, I'd expect to be able to do it from the side of the hole. I could lead to some bugs with several adjacent holes though. Then again, I was able to dig two holes at once already.
It's a bit odd for the guy to keep digging treasures to his last breath, never getting to use them. You could give the players an ability to escape with all the money they currently have or keep digging, losing it all if they die.