You have to realize all the stuff behind the scenes when hanging a new door kit. You have to carefully remove the old door, taking care to not damage the walls or the existing molding. You have to remove the door hardware for reinstall to the new door. You have to drill the new door holes sometimes, you have to make sure the door is level and square in a hole that in most cases is not square at ALL, you need to make sure it closes securely and tight. You have to paint it.
To install a door usually takes three to four hours per door IF there are not any problems which there always are.
All our home centers down here charge $250 for a door kit and do not install slabs only.
Being an independant contractor, I will hang a door for $150, and I have all that I can hang. You might consider consulting with a SMALL independant contractor and I am sure he/she will be lower.
Up front, there is nothing easy about installing a new door, this is your first line of defence to the outside environment and it needs to be secure and right.
If you have never hung a door kit, then this is not a project to learn on. Hire it out and you start on interior door kits.