Sums of money and periods of time

Use a singular verb with sums of money or periods of time, that is, do not let the verb agree with the head word of the noun phrase subject, but rather with the singular sum of money or period of time:

(1) [Ten dollars] is a high price to pay.

(2) [Five years] is the maximum sentence for that offence.

Even though both dollars and years are plural, we get singular agreement, since we are dealing with one (singular) sum of money in the first example, and one (singular) period of time in the second example.