A block has limited space: 4,000,000 weight units, which equals 1,000,000 virtual bytes (vB). Miners therefore rank transactions not by total fee but by fee per unit of space, the feerate.
How to calculate it
Feerate = fee ÷ virtual size. A 200 vB transaction paying 1,000 satoshis has a feerate of 5 sat/vB. Virtual size is weight divided by 4, so witness data costs a quarter as much as other data.
What drives your size
The number and type of inputs and outputs, not the amount of bitcoin. Sending 0.001 BTC and 100 BTC can cost the same.
Floors
Since Bitcoin Core 29.1, the default minimum relay feerate is 0.1 sat/vB (earlier versions used 1 sat/vB). Below a node's floor, it will not relay the transaction.