Why "pay top dollar" and not "pay top dollars"

I think the idiom started with advice by the State Board of Agriculture in New Hampshire, 1901...

It takes the bottom dollar to pay for the labor, the cost of production, the fertilizer, the taxes on the farm, and the mortgage and all that, but it is the top dollar that has the "funny business" in it. If you go to Europe with your wife next year it comes out of the top dollar.

Originally it didn't mean "obtain the top price" so much as "look to the 'margin' over fixed costs". Advice repeated in 1907 with similar wording by the equivalent Board in Massachusetts...

It is the only way to get the top dollar. It takes the first dollar to pay the rent of the land, and the next two or three for the labor, and the next dollar or two for the fertilizers, and another dollar or two for the spraying...

...and Vermont (but probably meaning, or at least implying, "top price" already)...

it is the breeder who produces the article always in demand that gets the top dollar for it.

...then by 1911 in trade publication The Poultry Item (definitely "top price" by now)...

I believe in every breeder getting every honest dollar he can out of his birds and if he has a specimen above the average in merit, to get the top dollar for him

It's not until 1931 that I find the first instance of "pay top dollar" (with the article "the" discarded), and even then we're still in the farming context, with American cattle producer: Volume 49

My customers thanked me for buying them preconditioned calves ... and will demand preconditioned calves and will pay top dollar for them.

Thus we see the dollar was perforce single when it started, and as the meaning gradually shifted, there was never a convenient point where it could be pluralised.

The phrase is often used in Britain too, but our own nearest "native equivalent" is pay top whack.

Imagine a stack of money. If you pay top dollar, you pay with that top dollar and every dollar bill under it in the stack. In other words, you pay a lot. It is an American idiom.

This idiom is also related to bet your bottom dollar. Same stack of money, but now you're so sure of something, you would bet it all from the top dollar down to the bottom dollar.