How the fee is constructed
The base fee is a fixed number of lamports per signature. The priority fee is a price per compute unit, multiplied by the compute units your transaction requests, and it is what validators use to order the queue.
Requesting fewer compute units with an accurate limit makes the same priority price cheaper, which is why well-built clients set an explicit compute budget.
What happens when it is too low
During congestion, transactions below the going rate are not included and eventually expire. There is no mempool holding them indefinitely — they are simply dropped, which is why they vanish rather than staying pending.
This is unlike Ethereum, where an underpriced transaction waits. On Solana it disappears and must be resubmitted.
Estimating a sensible rate
Clients query recent prioritisation fees for the accounts your transaction touches and set a price above the observed percentile. A popular account under heavy demand requires much more than a quiet one.
Overpaying is cheap in absolute terms, which is why most wallets default to generous estimates during congestion.
Fees are local rather than global. Heavy demand on one popular pool does not raise the price of a transaction touching unrelated accounts, because ordering is computed per account. A network widely described as congested may be entirely quiet for whatever you are doing.
What it does not buy
Inclusion, not success. A swap that fails a slippage check fails identically whatever you paid, and the fee is still consumed.
FBT Swap constructs Solana swaps through public aggregators with a compute budget and priority fee appropriate for current conditions, and shows the quote before your wallet signs.