Depends on what is wrong with it ? If it has a cracked panel or frame board, that might cost $150-200 or so to repair. If it is an in-production model, they might be able to replace the entire panel (if needed) for maybe $250-400.
If it got run into by a car, then there might be damaged rollers and track as well, which could significantly increase your cost.
Painting could be included if you so specify, though of course it would not be an exact match for the rest of the garage door - if you want that, then after it is repaired, get the entire door repainted - probably by a painter, not the garage door repair shop.
Insulated steel doors are generally (though not always) more energy efficient and airtight than wood doors, and of course need repainting less often, especially if yours do not have a protective second story overhang over your garage door to protect it from direct rain. Except for high-security ones, they are also lighter than wood doors, so it will make your opener last longer.
My opinion is that a double-skin (both sides) insulated steel door would be more sturdy and secure - they do not have wood inset panels to come loose, and are certainly harder to break through than a wood panel. They also do not require repainting every 5 years or so. However, they dent quite easily, so appearance can degrade easily. Do NOT get a thin-skinned door if you have kids under 20 or so - they WILL run into it with bikes, use it as a handball wall, lean against it and kick it with their heels, etc and leave dents all over it.
In general, unless the damage is severe or you are in a severe climate where energy efficiency has become a significant concern, I would think that repair would best fit your situation, as a new door could run $600-1400, depending on whether new tracks and opener are required.
One safety issue - if you do go with a high-efficiency door with good perimeter seals, and you have a furnace or water heater in the garage, be sure you have a fresh air inlet to provide ventilation and combustion air. Otherwise, your coal, gas or oil furnace and hot water heater could have inadequate air for full combustion, which leads to carbon monoxide. A very modern high-efficiency heater system may have fresh air ducting leading direct to it from the outside. Far more common is a simple 6 to 12 inch round aluminum or galvanized metal duct through an outside wall, properly near the top of the wall, with bug screening over it and connected to nothing at either end. When your door was put in, if it was relatively loose-fitting and without all-around seals, many builders did not put in a combustion air vent (even if required by code), but with a new door with all-around seals you definitely should have one. A sure sign you need one is when the garage door rattles or pulls inward on its rolles when the furnace kicks on. A plumber can put one in, or your local plumbing shop can figure the size you need and sell you the parts. They will need your garage dimensions (length, width, height) and the BTU rating from the furnace and water heater to figure the required vent size.