Besides the massive price difference of a 300 f/2.8, you're also forgetting at who and what they are designed for. You are really not even close to comparing similar lenses. Even if I could get one for the same price as the 70-300, it wouldn't work for me one bit. I have no use what so ever at this point for a 300mm prime, and neither would most people who choose the 70-300. It's not all about the IQ. For going out there and casually shooting the kids games, you can't beat a 70-300. Yeah, I may get better looking shots with the prime, but how many do I stand to miss? No 70-300 is marketed to professional or super-serious shooters. It's for soccer moms and people like me who need some reach on a budget, and it does it's job well. Unless you're willing to spend big bucks, nothing can really touch it.
PENTAX K10D
300mm 1/350th F5.6 ISO100
PENTAX K10D
300mm 1/250th F5.6 ISO100