I install storm doors for $80 and it takes abour one hour. But I have probably installed 200 of them.
They are not difficult, all you need is a hax saw, screw gun, and maybe a drill. Just remember one thing, measure twice and cut once and another think,
MAKE SURE THAT YOU DO NOT INSTALL THE STORM DOOR HANDLE AND STUFF RIGHT ON TOP OF THE INSIDE EXTERIOR DOOR HANDLE. IF YOU DO, THE STORM DOOR WILL NOT CLOSE BECAUSE IT BUMPS THE INTERIOR EXTERIOR DOOR HANDLE. MAKE SURE THAT THE HANDLES ARE NOT MEETING.
If you have not done one of these, then I would suggest that you break down and have it installed. My recolection of my first door was pretty nerve racking.