You don't say where your'e posting from but the following rule is fairly
international for a bespoke software developement that has no sell-on
value or you retain righst on finished product.
Work out how long it would take you to do in hours = X
What is the typicla hourly rate where you live for someone with your
skills and experience = Y
Material costs (CDs for burning, paper for manuals etc) = P
Expenses (travelling to and from customer site, telephone calls etc)= Q
Depreciation costs for your equipment, non-movable assets (e.g.
house/office) = R
Charge to Customer C= X*Y+P+Q+R
As it's a package you can estimate P,Q and R. You can also offer
discount of you think that this project can be a loss leader for follow
on work or discount of they pay up within a certain time of being
presented with your bill,
I would add one more factor: risk. By offering fixed price, you are
assuming the fairly substantial risk of estimating incorrectly. What if it
ends up taking 20 wks instead of 10?
For the same reason, it is critically important that you nail down the
exact scope of the project, and have a policy in place for charging for
any changes to the scope.
Also, when calculating the hourly rate, don't take the net rate of a
salaried person, but include the cost of benefits, taxes etc. that the
employer would usually pay. Or just use the hourly rate of somebody who
already works as a freelancer.