[delayed]

Yes, each opcode has a gas cost. Some are quite expensive, like writing storage (changing network state). Each block has a target gas limit. Say 30 million. A single transaction cannot exceed that. Additionally, a transaction specifies a bid on how much they are willing to spend, in ether, per gas. That said, transferring funds does not typically require significant gas.

What you suggest is possible (evaluate the side effects of the transaction and present that information to the prospective signer). But at present they don't do that. I'm not sure about this specific case but often it's just a supplied text string (that can say anything) that's displayed. Basically the system depends on trust in whatever came up with the transaction payload.

You can at least display the parameters that you're passing into a contract function. That keeps you from getting hacked when interacting with a well-known trustworthy contract.