First, the basic law now is you own the copyright.
Second, because you did it for work and worked on it at work they have an invested interest in it.
Third, your type of employment will have a bearing on who owns what, as do the state employment laws.

If they're offering to buy it, the easiest thing would be to make a deal where you continue to own the copyright and you sell it to them outright while retaining the right to use it yourself. That saves a lot of legal discussion about who owns what. Base the charge on how big it is and how many hours it took to create; use your current hourly charge rate.