What each side charges
A centralised venue charges a maker or taker fee on the trade and a withdrawal fee to send the asset on-chain. Internal transfers and the trade itself cost no gas because nothing touches a blockchain until you withdraw.
A decentralised venue charges a pool fee inside the price, network gas per transaction, and whatever the interface adds. Nothing is held, so there is no deposit or withdrawal step at all.
Where the crossover sits
For small amounts on an expensive chain, the centralised route often wins on pure cost because it amortises one withdrawal across many internal trades. For anything on a cheap network, the decentralised route usually wins outright.
For large amounts, depth decides it and the answer depends on the specific pair rather than the venue type.
The costs that are not money
A centralised account requires identity verification, is subject to withdrawal limits and freezes, and places your assets on someone else's balance sheet. A decentralised interface requires you to manage keys, and makes every mistake permanent.
Those are real costs on both sides. Which one you prefer to carry is a genuine choice, not a technical question.
An honest summary
If you value unattended execution, fiat on-ramps and deep order books, a regulated centralised venue does things a DEX cannot. If you value holding your own keys, no account, and access to tokens before any listing, a DEX does things a CEX cannot.
FBT Swap is the second kind: 0.70% of the input on supported routes, gas separate and paid to the chain, no deposits, no withdrawal fee, and no account.