I think it would be a pretty large range. Retirement just means you stop working, it's not an indication of skill level. For instance, you can create a standard 400 BP character and deck him out to be a grizzled old runner with a lot of jobs under his belt who's planning on retiring soon. Most retired runners don't end up as street legends. Your average runner will probably retire once he amasses ~ $650,000, which is enough for a permanent medium lifestyle for a family of four. Add gear, implants, vehicles, etc into that and you can round off on an even million.
Then you get the crazy bastards like Fastjack who can easily afford to retire and probably should, but keep hacking away anyway because they love what they do and they're extremely good at it. The old fart probably has over 1000 karma. Or Warren Buffet in real life, who just reached octogenarian status and is a multibillionaire, but runs his business anyway because there's nothing else he'll rather do. Then again, being a CEO of a company usually doesn't involve getting shot at on a daily basis, so you probably don't see many 80 year old shadowrunners running around.
As a rule of thumb, you can always take a tip from Missions, which "retires" characters at the 150 karma mark.
