Inserting data into blank & non blank cells at same time

I have a group of cells that I want to add a numerical value to.
i.e. I want to add the value $5.00 to cells A1:A4
A1,A4 have no number in it so their final value should be $5.00 per cell
A2 has $2.00 in it so it's final value should be $7.00
A3 has 1:50 in it so it's final value should be $6.50
I would like to bev able to select all the cells at once and enter the constant number.
Any ideas?¿