You can not add fees that were not part of your original contract. You are owed any wages that you were not paid. I am only licensed in Massachusetts but in Massachusetts there are statutory penalties if wages are unpaid.
If you are owed unpaid wages you should contact a local employment attorney who can do a detailed intake during a consultation. They will then be able to get all the facts surrounding the situation and help you determine how you should move forward from this point.
You can search for a Lawyer using the AVVO "Find a Lawyer" tab up above or contact your local bar associations.
Unless you had a contract that provided for fees and interest, the answer is that no you cannot just come up with some figure. If you don't have a contract, please speak with a local attorney to draft one that includes these terms in addition to venue and other clauses to protect your rights and business.
Now, if they refuse to pay you may consider filing a lawsuit. If this is $7,500 or less than small claims would be the best route. See the information here http://www.courts.state.co.us/Forms/Forms_List.cfm?Form_Type_ID=237. Good luck on this and speak with an attorney to avoid these kinds of problems in the future
This answer is for informational purposes only and is not legal advice regarding your question and does not establish an attorney-client relationship.