I kind of doubt this is useful.
When command abort due to error, they usually set no cost.

What is your usecase? In what case do you want to get cost, when the operation does not succeed?

The AI queries for the cost of cloning a vehicle in test mode, to get how much it would cost, so that it can borrow the necessary money from bank. It was expected to get the cost, but CmdCloneVehicle returns failed for a command that was supposed to be run in test mode. Even though the cost is already computed, the AI received a £0 as estimate, and once it borrows the 'necessary' money, it tries to clone it in exec mode (and fails, because £0 was the wrong estimate).

The comment in vehicle_cmd explicitly states that cost estimates do not work for cloning.

This is the case for all commands with CMD_NO_TEST. Returning a wrong cost does not help either.

In the more general case the codechange is also wrong: when there are errors the cost is not guaranteed to be meaningfull in any way, which is exactly why it's fixed to 0 for AIs.

This could use an update to the AIVehicle.CloneVehicle documentation to let AI authors know there is no cost estimate available and what would be the alternatives (likely manually checking EngineIDs and get build costs for those).

@frosch123 In GUI, the tooltip for cloning vehicle says shift-click to get estimated cost. From a player perspective, I get an error and the required money as the 2nd part of the error message. Why limit that for AI's?