Local SEO/ Multiple Location Issue

[localdominator]

I am focusing purely on getting companies listed in GP and then referring all natural seo to others. I have been running into the problem of some potential clients who want to do literally a few hundred listings across the US if I can solve the location issue. Since virtual office/ups suites are not very cost effective, I was wondering if you have a suggestion. I don't want to black hat the locations(already familiar with how to do that), since these will be long term clients. I have the phone issues solved.

Your insight would be appreciated.