angelitos,
If I understand you correctly, I think all you need to do is to:
1) Go to the cell location where you want the "constant".
2) Enter a simple formula that refers to the original cell. If the original formula is in cell "A1", for example, then enter this formula =A1. To make it "constant" (or "absolute"

, you need to enter a the "$" character before the column letter and before the row number - i.e. =$A$1.
An "easy" method of referencing another cell - in an "absolute" way, is by first assigning a RANGE NAME to the cell. Then in your other cell, simply type =costs - where "costs" is the name you assigned to the original cell.
To assign a Range Name, used the following steps:
1) Highlight the cell.
2) Hold down the <Control> key and hit the <F3> key.
3) Type the name.
4) Hit <Enter>
Caution: Don't create Range Names that conflict with Excel's cell coordinates or with numbers. For example, don't create a name such as "C14" - instead use "_C14".
If you have another case where you want to assign a Range Name to a range-of-cells, as opposed to one cell, this is also possible and you would use the same Range-Naming procedure. Then, for your formula, you could use another function such as =SUM(costs). Whenever a formula contains a reference to a Range Name, and the formula is copied to another location, the reference to the named range will stay “fixed” or “absolute”.
Hope this helps.
Please advise as to how you make out.
Regards, ...Dale Watson dwatson@bsi.gov.mb.ca