DB work is not in my scope of work and my contract is deliverable-based so it's actually a free product the client is getting from us.
You say you are providing the work - but the client is paying your employer for your time so they are still paying. The relationship between what your employer charges your client and what it costs your employer to support the contract are pretty much unrelated.
If you did not do this work then there are a number of scenarios (there may well be others)
a) you would spend less time there so your employer would charge the client less. Depends on the contract
b) you haven't got enough work to do for what your employer is charging the customer.
c) the fact you are doing this work 'foc' helps you employer to keep the contract.
d) if you are working into the wee small hours to help the client - get a life
e) your customer would have to sub this work out - which I guess is what you are trying to do is estimate what is the cost of doing this?
Simply - there is no standard pricing. Suggest you visit one of the websites that provide an arena for potential customers to contact developers. It comes down to what the customer is prepared to pay and the willingness of the developer to work for that rate.
Here is an example
https://www.odesk.com/o/profiles/browse/skill/microsoft-access/
You will find a wide range of developer prices and abilities.
An expert developer may take a third of the time to deliver a project over an inexpert one which will have an impact on the overall cost.
As far as contract rates are concerned (i.e. what a customer is prepared to pay), in the UK, these vary from just over the minimum wage (circa £50/day) to perhaps £700/day - the average is probably between £200 and £500/day and is often wrapped up with the requirement for industry experience and the ability to work with other systems, people skills etc. You also need to take into account who's equipment you are using, travel costs, who owns the rights to the development etc.
To price by form/report etc., how complex is the form? is the data readily available? etc. without knowing these things, it is anybody's guess.
If you are talking about comparing yourself with solutions developed on SAP, SFDC, etc then another factor from the customers perspective is ongoing support - their position on this is probably much more important than the cost. They are generally not too excited about a solution that is at risk of being unsupportable in the event the developer gets run over by a bus. Nobody gets fired for spending £1m on a system that is an industry standard.
Both Plog and Mihail are right - but where to start with a number? One suggestion is to take your salary as an hourly or daily rate and multiply by 4. Another is to take your employers chargegout rate. Both are reasonable guesses that your employer can relate to. To find out what a 'database support' company charges is not viable, tho' you may find some headline rates out there if you google 'access support' or similar.