I sure someone can help out with an elegant solution to my problem...

I am constructing a spreadsheet to help monitor a small business. It is
planned to be a combination of a budget spreadsheet and a reporting
spreadsheet. I have two columns for each month one titled Budget and the
other Actual. So I have columns Jan Budget, Jan Actual, Feb Budget, Feb
Actual etc. And each row is a budget item, say staff costs, material cost,
rent etc. etc. At the start of the year there will be no actuals filled in
but as the year progresses actuals will be entered.

My problem is that when I sum either vertically for a month or horizontally
for a budget item I would like to use the Actual figure if there is one and
the budget figure if not.

I can do it if I set up a helper matrix of zeros and ones with a very clumsy
set of logical tests or if I use a whole string of horrible "IF" statements
but I am sure this is not the best approach. Surely there must be a formula
(using SumProduct???) that could help me out.

I hope I have explained my problem clearly enough. Any pointers would be
greatly appreciated. Thanks in advance and apologies for such a long post.

Peter