cost to repair - this is a tough question without knowing all the facts. you didn't say what kind of car - year, make and model, and where you are at - north? south? canada? or even you may be in europe. this is an open forum, people are from all over the places. so you know you left a big door wide open for imaginations... generally speaking, the cheaper cars are the cheaper cost to repair will be; high-end cars go to the opposite direction. the labor rates at different areas play a big role too. just to give you an idea, take an example of a cheap toyoto echo, a single door repaint could be around $300 in the middle of usa, say tennessee, by a pop-mom shop. a mercedes e350 in san jose could be more than $1000 by a dealer. go from here and make your guess for your specific case.
what to do immediately - report to your local police. it will not help you find the person who did for you, it will keep police department informed there is a vandalism in the community. they may catch the guy in long run, most importantly you are helping your community and making people aware of it.
what are your options - there are many factors affecting your decisions, just like anything else, to best fit individual needs/goals. per your tendency of perfectionism, level of tolerance, depth of pockets, social status, job nature, family attitude... the actions could be taken differently. here are a few choices:
1) fix it by a shop - get quotes from bodyshops
2) live with it - a lot of people do so, i am one of them. when i bought my 2010 honda fit, the right rear door had already been keyed and the frontend been damaged, but it had only 16000 miles on the clock otherwise it's in a good shape. in that year the dealer price was $16000 otd, i paid $7300 from a private party. knowing i was to get a little car for towing i really didn't care much. i fixed the frontend which cost me less than $1000 for parts, and it has been good ever since.
3) do a cosmetic fix - this was what i did and it worked - find a fine sharpie with the same or similar color; draw along the line/ink it; use your finger to wipe out excessive ink while drawing. it will effectively remove the reflection of the line and will stay for a long time. on my car, passers-by will not notice the lines beyond 10 feet away.
bottomline - whatever you do, be happy. good luck!