How would I determine what to "Sell" my Developed Applicaton for?

I am a financial consultant and make my living "by the hour" providing consulting services.  I have developed an Access Database using Access 2000 which enables a Company to automate the process of documenting their internal control structures.  I have been approached by an acounting firm (the "buyer") that wants to use the Database in their firm of about 200 people.  About 40 people would actually have the application on their Laptops.

The buyer has offered:

1. To use their IT staff to migrate the Access Database toSQL and enable Web Access using ActiveX (or WebX) controls.  MS Access would continue to be the front end.  I was going to outsource this development using my own money and found it would cost around $15,000 - $20,000.
2. They would NOT own the software - I would always own it, and the intellectual property, and be able to use the newly improved version for my own consulting engagements on other clients.
3. They would pay me a retainer of sorts to provide training to their people and to modify the software as needed.  The "contract" would be for a year and basically say that they will pay me my hourly rate at say a minimum of 8 hours/week.  
4.  If a "major' tweak needed to occur, I'd price it out acordingly based on the time estimate.

My question is this - Am i getting screwed, or is this typical in such an arrangement?
Can anyone direct me to any websites that gets in to agreements of this nature?

Any help would be appreciated.

Thanks folks!!