Whirlpool is different than other CoinJoin services, in that you do not pay a volume based fee per anonymity set, but instead you pay a one time flat fee for an unlimited anonymity set.
To put it simply, it costs the same in Pool Fees to cycle 1 BTC or 1000 BTC. Once the pool fee is paid, it costs nothing to continue cycling. With each cycle you gain a greater privacy advantage with a deeper anonymity set.
Choosing the right pool
The denomination of the pool determines the minimum amount you can cycle through that pool and the resulting denomination of the newly cycled UTXOs in your wallet. Generally the larger the UTXO you are cycling, the larger the pool you want to choose.
Cycling 1 BTC in the 0.01 Pool would create 100 outputs of 0.01 BTC
Cycling 1 BTC in the 0.05 Pool would create 20 outputs of 0.05 BTC
Cycling 1 BTC in the 0.5 Pool would create 2 outputs of 0.5 BTC