has/have

Three thousand dollars have been spent on this project.

Three thousand houses have been damaged by storms.

Three thousand people have bought tickets for the concert.

Three thousand votes have been cast in the election.

Thousands of dollars have been spent on this project.

Thousands of houses have been damaged by storms.

Thousands of people have bought tickets for the concert.

Thousands of votes have been cast in the election.

Plural, therefore "have"

-- but ---

This may be the source of your confusion:

The sum of three thousand dollars has been spent on this project.

A sum amounting to thousands of dollars has been spent on this project.

A total of three thousand votes has been cast in the election.

Three thousand is greater than one - it is a plural quantity - but one sum of three thousand dollars is a singular quantity.

Thanks, Contrex.