Write your program out in solidity. It’ll tell you the gas cost, you can then calculate what that would cost at varying gas prices.

Yes, scam.

This is an obvious scam.

Just a note, there are lots of scammers out there that will try to convince you they want to buy your art as an NFT and they’ll insist you use some website of theirs that has lots of fees associated with it. They’ll keep telling you the next fee is the last fee and then they’ll buy, but it never is. Be careful for this type of scam. Look at /r/NFT, they have people asking about this kind of scam many times every week.

It’s fine if you do, but relying on that as your only means of backup is a significant risk.

Sounds like you fell victim to an address poisoning attack.

In an address poisoning attack, a scammer looks for transactions on the blockchain and identifies wallet addresses. They then create a new wallet with an address that is very similar to the one used in the transaction, usually differing by only a few characters. The scammer is betting on the fact that when you make another transaction, you might not double-check the entire address and end up sending your funds to their similar-looking address instead.

These attacks exploit the user’s oversight and the complexity of wallet addresses. To avoid such attacks, it’s crucial always to double-check the entire address when sending or receiving cryptocurrency and to use address book features in wallets that allow you to save and label trusted addresses.

Your funds, unfortunately, are irretrievably gone. You could consider filling a police report, but police aren’t helpful with most small crimes and even less so with cybercrime so that’s a decision you should make on your own.