Front door s/o: screen doors.

[SusanBAnthony]

Our new house has a standard builders grade front door, and back door (leading to the deck).  Neither have screen or storm doors.  I would love love love to have a screen door on both, as we typically only run the a/c rarely, and I like to be able to hear the kids outside, and for them to be able to go in and out easily without worrying about bugs.  

Can you add a screen door to an existing door?  Is it an easy project, or should we plan professional installation?  Any idea on price?

i imagine we will eventually replace the boring front door, but it is low on the list of cosmetic upgrades, so I don't really want to wait around for that.