Assuming dates in column A (A1 to A1000, say), and costs in column B,
enter January, February, March etc to December in C1:C12. Then enter
this formula in D1:

=SUM(IF(MONTH(A$1:A$1000)=ROW(),B$1:B$1000,0))

Do not use <enter when you have typed it in - use CTRL-SHIFT-ENTER at
the same time, and if you do it correctly then Excel will wrap curly
braces { } around the formula. Copy it down to D12.

Hope this helps.

Pete