Hi Lucy,
Not automatically. A macro could be written to do this. But if you don't mind using a seperate column, you could, in the next blank column write a formula that references the first to do your markup.
For the sake of illustration:
Row one contains titles of your column data
Column A contains the data you want to mark up.
column B is available for the formula
The first formula in cell B2 would be:
=A2*1.05
After entering that formula, copy it down for many rows as you have data in column A.
Lucy, you could temporarily enter 1.05 into an
empty cell, Copy it, select the prices that you
want to revise, and choose the Edit | Paste Special...
menu command clicking the "Values" and "Multiply"
radio buttons.
Also, there's a popular free utility, ASAP Utilities,
that supports such transformations. See
http://www.asap-utilities.com/ .