The crook roofer up the street wanted to charge me 6700 bucks. He wanted to sell me a ridge vent and the fancy dimensional shingles and all that nonsense. I paid my 2 helpers 400 bucks each the dumpster cost 600 and my materials were 1500. 2900 total. I went with the 3 tabs, I didnt want a work of art, just a friggin roof. If I ever have to put a roof over another one I may go with the dimensionals since if they aren't laying perfectly flat you really cant tell.
When we tore the old roof off, the plywood (30 years old) looked perfect. Ventalation isn't a problem with the gigantic attic fan and gable vents they installed when they built the house. I slapped a brand new one in though, they are only 54 bucks at Lowes and you have to shingle around it so I figured I may as well replace it while I was up there.
We did an old lady's roof a while back and the plywood was crap. When we told her, she didnt care and told us to shingle over it. We were pushing the nails in with our thumbs.