The changes tht occur in your credit rating can vary. If you have already missed a few payments, more than likely your credit score has already dropped. Having a bankruptcy or a foreclosure on your report is much worse than a short sale. A short sale will show as "Paid In Full - For Less". Depending on other items on your credit report, you could see as much as a 30-50 point drop due to the short sale. It could be as low as 10 points.
It does not come close to the drop you can see when a foreclosure is recorded. That has been known to drop credit scores as much as 300 points!