The effect of a PR Manager is minor. Basically if you are moving up levels to a higher PR you need to justify it. I was able to buy a PR guy for $9000 with a $10,000 salary as advanced. However, I don't think he has done much. But I got him for such a good deal I figured he was worth keeping around for a couple seasons.
My best estimate is that the PR guy has an effect of about 3-5% per level on both ticket and merchandise. Also many people have come to the conclusion that it helps more with a losing team than a winning team.