I'm not trying to be a d**k but i can see it from your friends dad's point of view.
PC shop, mans invested a lot of money and time and does this for a living (even if he is ripping people off, most of them do).
You - kid. (sorry)
I was doing just the same as you when i was your age and had the same problems with parents who thought the tower of a computer was the hard drive.
His dad would rather have the security of paying over the odds and letting mr pc man build it rather than you. Building PCs isnt hard, getting some one trust to you with there hard earnt cash is. Good luck, but i susspect you need to convince him of your abbilities to build it rather than the specs.
This is what happened to me when I was around 14 years old and people would ask my dad to build it for them, and he would always mention that it was me that would do the building etc and they always looked at him, as if he was out of his mind.
When they then saw my pc in photos or at the house they changed there minds every time.
Where I live we have a small computer shop that is ran by 2 guys, and I went in there one day and asked for a price on a system with watercooling etc, just to see what the price would be and if it was £20 more than where I normally buy my stuff from then I wouldn't have said no, but when they came out with a price that was £450 more I just laughed and walked out.
The issue with shop's that are small and usually ran by 2 or 3 people from the same family is that, because they generally don't have that much business they struggle to get prices for components that other retailers can get due to, bulk buying discount etc so those little shops will always cost more.