1) Kellogg is definitely worth the investment, particularly if you have a marketing career focus.
2) remember that average GMAT scores are just that--the average. MAny students get in with below and above the average. what you need to do, it to demonstrate value in yourself as a b-schools student, with evidence that will compell the committee to select you despite a marginally lower score over someone else with a similar background.
3) PT programs are typically more forgiving of GMAT scores not only because they realize you are working full time and therefore do not have as much marginal time to devote to test prep and 2) the stats for their part time program do not figure into their FT rankings. Schools only submit their FT program stats to the rankings boards. So the business week and US NEws rankings do not include PT stats. This allows schools to give a little more grace on the admissions requirements for the PT programs.

I'm actually in a semi-similar situation. I'm interested in attending Kellogg Part-Time, just re-took the GMAT (1st time 660, 2nd 600). I don't want to retake it as I'm not a great test taker. I have a 3.5 GPA, 8 years Marketing Experience. Do you think my 2nd GMAT score is going to severely hamper my chances of getting into the Part-Program at Kellogg?

Poosibly not a problem--660 is a good score, just not in the top for Kellogg. It's likely in the 80% range, however. Part time programs are more forgiving on things like GPA and GMAT for several reasons. The biggest one is that they don't report the stats for their parttime programs to the rankings groups, who tally scores for the full time programs. for this reason, there is less risk in having a lower GMAT average among the part time admits since it won't "damage" the rating for the flagship (fulltime) program at that school. Additionally, part time programs understand that applicants don't have the same amount of time to spend preparing for the test, so the results are not often reflective of the applicants true abilities.