strange results when adding negative figures in formulas

This might be a very simple one, but I've been so busy lately that my grey matter has hit a slow moment. For some reason when tallying prices, if I hit a negative figure (-$500.00 for example), then next sum down is incorrect and I have to re-insert the formula.
Is this a common bug with Excel, or is it the nut behind the keyboard? Do I need to input a different formula in order to deal with minus amounts?

The sheet I'm using adds as you progress down the column. The input figures have been calculated from a similar previous column. Usually the comparison between the two will be a positive figure, but sometimes it ends up as a negative, and this is where the fun begins