What a revert undoes and what it does not
A revert rolls back every state change the transaction attempted — balances, allowances, pool reserves. It does not roll back the computation itself, because that already happened, nor the nonce in most cases.
If it were free to fail, an attacker could flood the network with transactions designed to revert and pay nothing for the load.
How much a failure actually costs
Usually less than the successful version would have cost, because execution stops at the failing check rather than completing the whole route. A slippage revert on a swap typically burns a meaningful fraction of the full cost, not all of it.
The exception is out-of-gas: that consumes the entire gas limit you set, because execution ran until there was nothing left.
The repeat-failure trap
Three reverts in a row on a volatile pair can cost more than accepting a slightly wider tolerance once would have. Conversely, raising tolerance to force a transfer-fee token through converts a free failure into a permanent loss.
The deciding question is always which of the five causes you are actually hitting — the guide on failed swaps lists them with their signatures.
Nobody can refund it
Gas is paid to validators and partially burned. It never reaches the interface, so no interface can return it, and a service that offers to refund network gas is either subsidising you from its own funds or is not telling the truth.
FBT Swap re-quotes immediately before signing, which removes the stale-quote revert — the single most common avoidable cause.