You would have paid part of that 21 million to miner fees, and there will never be 21 million bitcoin anyway, so not that much.

However, assuming you have a channel directly with who you're trying to pay, you can pay the full size of the channel. If there are hops that will be a very difficult thing to calculate because I would have to know the channel sizes of the nodes routing your payment, the fees they have, assuming a multi-part payment, what sizes all channels along all paths have to contribute and all of their fees.