atw: selling a team logo copyright

Anybody have any experience with selling the copyright for logos that you have created?
It has not been an issue before, as I have created the team logos (as well as graphics) while on contract as the Technical Writer / consultant, so everything I did automatically became the property of the company to whom I was contracted at the time.
I drafted the basis outline for this team logo when I was still on contract to the company and they did nothing with it at the time. Now they've contacted me and asked me to submit some additional variations on the logo and want to know what I want for the additional time and the copyright on the one that they choose to use. Calculating time is no problem, but I have absolutely no idea what the going price is for a custom designed team logo.
BTW: This is not an overall "company/corporate" logo - which would be a much bigger deal. This particular corporation has "teams" that are dedicated to various internal and external projects, which have project names, and identity through individual logos. Team identify is used in e-mail, advertising to "customers" and on T-shirts. I have already created another team logo for a different team, while still on contract to this same company. As such, my time was paid, but copyright ownership automatically belonged to the company.
Side bar: While I don't plan on working for free, I also want to remain in the good graces of this company. 
Mary

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