Even the "experts" can't seem to figure it out.
Taking the 8 year average is meaningless, it's like taking 4 apples and 4 lemons - and then saying on average, we have a lemon.
Look at what actually happened, including the Padres offensive stats in petco
2004- 2008, Padres payroll in the 65 - 75M range:
.246/.322/.379, overall WP: 0.504, home WP: 0.526, road WP: 0.483
All very reasonable numbers, 3 postseasons
2009 - 2011, payroll < 60M :
.227/0.310/.345, overall WP: 0.487, home WP: 0.504, road WP: 0.469
All horrible numbers, 0 postseasons
There is an extremely obvious correlation here, more money = better team, all the while the park stayed the same (besides the fences that were already moved in).