...but 20 dollars is 20 dollars [duplicate]

There is a well known phrase: "I'm not gay but 20 dollars is 20 dollars".

Why do we say that '$20 is $20', and not '$20 are $20'?

It's stressing not individual dollars, but an amount of $20.

You might likewise say that five gallons is not a lot of fuel, that five years is a long time, even (in the right context) that five people is a small team.

Understand that what's being talked about here is not a number of items or a quantity (whether countable or not) but a metaphor of sorts for some principle.

If a screwup by your doofus nephew caused 587 turkeys to suffocate in a closed railway car, and your aunt was trying to convince you to "forgive and forget" vs somehow making the nephew compensate you for the loss, you might reply "That's all well and good, but 587 turkeys is 587 turkeys," meaning that you've still suffered a tangible loss that was his fault, not yours, and he should have some responsibility for it.

In this case "587 turkeys" is not (rhetorically) a quantity, but rather it's a metaphor of sorts for the tangible loss. You could have (a little less dramatically) said "but a loss is a loss" or some such.