Ad a new, temporary column (or one you can hide when not using). Let's say the current Prices are in column C and the new column is D.
you can inflate the prices in column C by 5% by entering the formula in the first cell of column D as:
to fill down, select D2, then copy, then select the D3 thru the end of the column, then paste. Now select C2 thru the end of column C, now paste over the existing values using the menu item "Edit > Paste Values"
you can now hide column D until you need to use it again.