Staffa - 8:37 pm on Sep 1, 2008 (gmt 0)

Maybe try this : imagine a lump sum for the big project proportionate to what you would charge for a smaller project. Then estimate as best as you can the number of hours that would be involved to do the job. Divide the lump sum by the number of hours and see if cost/hour is a value you want to work for.
Then adjust for any extra costs/expenses.

It's not perfect but it will give you an idea.