Sentence Correction

Hi,

I'm pretty new to GMAT so please forgive me if this question is too elementary. The below sentence is from the OG 2019 on the subject of agreement.

"Six dollars is a high price for that"

I'm of the opinion that "Six dollars" is the subject but I can't wrap my around why the verb is taking a singular verb-number. I appreciate that it won't make sense to replace the verb with "are" but I cannot articulate why the subject is plural while the verb is singular.

Welp!

Dollars can be put under 2 cases -

When used as an amount of money, it requires a singular verb
Five dollars is a lot of money.

But when referring to the dollars themselves, a plural verb is required.
Dollars are often used instead of rubles in Russia.