[Ethereum] Price of 1 gas in dollars

The total storage is made of 32-bytes slots addressed with 256 bits. This gives us 2^256 * 32 bytes to use. When you add an item to a mapping, it is sent to a random location in the storage calculated by sha3, see this answer.

Adding an item to a mapping will never fail because there always will be a sha3-calculated location to put the info into. Of course, the closer you get to 2^256 insertions, the higher the likelihood that you will eventually overwrite something else.

I think you got something backwards... you set a gas limit of 42000, which was evidently insufficient. You should pretty much never adjust the gas limit, as it's very accurately determined ahead of time. MetaMask should be getting the gas limit right for you.

You also set a very high gas price: 250 gwei, when https://www.ethgasstation.info/ says 80 gwei should get your transaction processed within 2 minutes.

No, there's no way to get back the ether you paid for that transaction.

In the future, let MetaMask choose the gas limit, and set the gas price by checking https://www.ethgasstation.info/.

EDIT

I should note that an out of gas error can also occur when something else goes wrong in the transaction. Some kinds of errors can cause all the gas to be consumed. (E.g. an assert in the code.)