I can agree that developing all Contacts as fully-built Prime Runners is, for most GM's, a non-starter.
As to determining DP and keeping it consistent, yeah, it's something the GM conjures up with when he needs it. To keep it consistent, he should write it down after he comes up with it.
Game example:
My team met a fixer who's intended as fairly influential and a lot of his connections are corporate types, I already know from his role in the game that he's got a Connections rating of 5.
So, the first time the runners wanted him to buy something for them, I thought to myself, "He knows a lot of important players and gets along with them despite them knowing that tomorrow, he might be dining with their corporate enemies, arranging a heist". From that, I knew he needed a good Charisma score, let's say 5. Negotiations is fundamental to his job, day in and day out, he's talking his way through one deal or another and keeping all his competing interests from begrudging him, sounds like a 5 or a 6, and possibly a specialty, but not in shopping.
So He's got Charisma 5, Negotiations (Corporate +2) 6, and Connections 5. That's a Dice Pool of 16 (18 if he could justifiably be trying to buy from a corporation), plus/minus social modifiers (he's well known, so he probably has a Street Cred bonus in there, too. Another minor detail decided by the GM) when he goes shopping. That DP won't change unless there is a story reason why he became more/less influential, charismatic, or well known.