How do I make Excel stop treating blank cells as zero? Check out the following grid and my explanation, please.

Beginning Cost.........FinalCost ...........Savings
194............................................... ............ 194
286 .................................................. .........286
367 ................................315 ...................52


........................................Total Savings: 532


Column A is the initial cost, while Column B is the lower cost that i've negotiated. As negotiations are done, I'm updating column B with the "final cost." Column C, "Savings," simply computes the difference between column A and column B. At the bottom of Column C, I have a cell to compute the total savings using a basic "Sum" function. The problem is I want to keep a running tally of how much we'ved saved thus far and the fact that Excel is treating the blank cells as zero means that it's giving me inaccurate data. It appears that we've saved $532, when we've really only saved $52 thus far. I update Column B (Ending Cost) as that data becomes available. I know this should be a basic thing to do, but I've looked everywhere and haven't found the answer. Thanks in advance for your help!