If an interior door is damaged, replacing it without removing the trim and hinges will save you some money. You would pick up what's called a "slab" door (without knob, strike plate hinges, etc.), and here's how you would install it.
Things you'll need to buy: Slab, Knob Drilling Kit, Knob / Strike Plate Kit.
1.) Measure it. A standard interior door is usually 30 inches wide, but measure the existing door to make sure of the dimensions (width and height). Pick up a slab at your local building supply store that matches these dimensions. Once you get the materials home, there will be a few things you'll need to do to prepare for hanging.
2.) Recess the hinges. First measure from the frame down to the first existing hinge, making sure you create the hinges on the new door the same distance. From there, draw an outline of the existing hinge, and either chisel (with a flathead screwdriver or chisel and hammer) or use a router with the appropriate end to dig out about 1/8" to 3/16" within the outline you drew. Then hang it on the top hinge, and mark the other 2 hinges down the side (this way, you don't have to measure the distance for the remaining). Follow the the same recess procedure for the other two.
3.) Hang the door up and ensure it closes. If it seems to be a "tight close," you may need to chisel out a little more to loosen it up a bit. If it seems really loose, you can cut card stock to the size of the hinge and place it in between the hinge and the door.
4.) Drill holes for the knobs and striker. Hang and shut the door, then mark where the strike plate on the frame meets the door. This is where you would use your knob drilling kit. Basically this kit is a template that fits on the door, and includes instructions that walk you through drilling the holes for the knobs and latch. Mark the center of the strike plate, line up your template, then use the included hole saws to first cut the hole for the knob, then the striker.
5.) Install your knob (striker first), following the instructions on your knob / strike plate kit. Ensure the knobs work correctly, and you're done!