Generally I think most target scores are driven by a couple of different things:
1 - Target school/programs: This is probably going to be the #1 driver in what folks consider when coming up with a target score. If you are looking a specific school and program the average GMAT for said school/program is going to be a big factor in determining what score you want to hit.
2 - Performance on initial practice tests: Most of the experts (and I agree with them) will tell you to take a practice CAT early in your prep to get a baseline of where you are starting from. I know for me this coupled with my target school informed what score I was targeting. That being said I started with 500 on my initial CAT and scored a 700 on my GMAT so it doesn't mean that a lower starting score has to limit your target. That being said, on to factor 3....
3 - Prep Timeline: If you have a set timeline in which to prep for the GMAT that can play in to what score you target. Conversely your target score may dictate your timeline especially if you are dead-set on a specific school. For example if you want in at HBS and you have a baseline CAT of 520 you are going to want more than 2 months to prepare so you can get your GMAT score up around the school average of 720 something.
4 - GPA/Work Experience: Depending on your undergrad GPA or amount/type of work experience you may want to target a higher GMAT score than a school average. This is to generally help compensate for a lower GPA or limited work experience.
5 - Personal Gratification: I have to add this one because it was a big part of my motivation. I believe my publically stated GMAT target was 650 as the part time program I was applying to isn't too GMAT focused in their assessment. While I would have been happy with the 650 the competitive part of my want that 700+ score. I set my target lower because at the end of the day I didn't need to put extra stress on myself but in the back of my mind I was still gunning for that higher score.
Hope that helps! I'm sure others will have more to add or dispute my comments
