Get prices for everything, get numerous bids, get references, go look at their work.
After you did this for every single sub you would have wasted enough time to more than make up for the Builder's Fee.
Like I mentioned earlier, unless you have A LOT of free time and/or relationships with subs somehow already, it's going to take you a ton of time to do a half way decent job.
Getting quotes does take some time. But not that much. I was home recovering from hand surgery and started making calls. I bid out everything. I mean everything and it took about a day or two.
I will agree with you on one thing. Having a relationship with the subs does make it easier. My old man is a plumber that does alot of residential new construction so I got alot of quotes from guys whose work he was familier with. With that said I did not always use the lowest bidder and also used guys that I did not know and the only problem I had was with my foundation diggers.
Like earlier posts said never pay in advance and hold the subs money until the work is done to your satisfaction.