Are your values in your drop down custom values or are they values stored in a related table?
If you store the values "1/4 page, 1/2 page and whole page" in separate records in a table, a second field can store the corresonding values of 0.25, 0.5 and 1 then a looked up value can be set between this table and your current table to copy the decimals when the matching text is selected.
Another option is to define the "space" field as a calculation field:
case (advert display size = "1/4 page" ; 0.25 ; advert display size = "1/2 page" ; 0.4 ; advert display size = "Whole page" ; 1 )
Many thanks Phil