Is there such a thing as a "constant" cell

I'm sure I'm asking the question wrong, so please pardon my long winded description. I've always appreciated the way I can copy a formula field in excel from one cell to another, and that the program automatically modifies the cell locations to fit the copied location. So, for example, a cell that is specified as "=SUM(A1:A30)" to produce a total in cell A32 can be copied to B32, and it now becomes "=SUM(B1:B30)". How convenient! Even of I later insert or delete rows between rows 1 and 30, that formula adjusts. But where this feature gets in the way is when I DON'T want i to apply to a particular referenced cell. Lets say for example instead of a sum, I want a percentage of the sum. And now lets say I have a cell somewhere in the spreadsheet, perhaps at A50 where I have placed the percentage I want to calculate. So I put "0.10" in A50 to signify 10%, and I alter my initial formula in A32 to now be "=SUM(A1:A30) * A50". This works of course, but if I now copy the formula in A32 to B32, its going to become "=SUM(B1:B30) * B50", which fails because there is nothing in B50, and I don't want to have to populate each column in row 50 with duplicates. Of course I could edit each copy, but that's a pain too. I don't think "constant cell" is really the right description, but I what I'm looking for is a way to express that a cell specified in a formula should not have its location (vertical or horizontal) altered if the cell is copied. Of course I realize the the convenience of being able to insert or delete rows from my original scenario might be lost if this were possible, so maybe a better alternative would be a way to designate the cell itself (A50 in this case) to NOT have its location altered if ANY formula that references it when the formula cell is copied.

Hopefully this makes sense to someone. :-)

Thanks everyone! So '$' is the magic designator. Much appreciated.