Celebrities don't always cost much, you have to remember that those high rates they get paid are optional, there's a base pay for actors, and they can take that rather than their typical high pay if'n they want. Throw in that in a lot of ways voice acting is easier (no costumes, make up, no standing around for a scene to be set up, etc), and it's effectively a quick and easy buck for actors.
Celebrities don't always cost much, you have to remember that those high rates they get paid are optional, there's a base pay for actors, and they can take that rather than their typical high pay if'n they want. Throw in that in a lot of ways voice acting is easier (no costumes, make up, no standing around for a scene to be set up, etc), and it's effectively a quick and easy buck for actors.