I've always thought that you could steal as much as you want, provided that it was still within robbing limits.
You can rob MP5s, Desert Eagles and AK-47s that're easily worth more than $5,000 so I think you can rob :
- $5,000
- 500 Materials
- 10 litres for Moonshine, 15 grams of any other drug
- MP5, Desert Eagle, Shotgun, 9MM, AK-47, Shovels, Bats, Golf Club, Flowers, Nightstick, Dildo's + Vibrators
Source.
And no, like Duff said, you have to divide the limit between your accomplices.
Note: It is under our discretion to bring as much things as we want; that's why there're post offices, cars, houses and RPly stored briefcases in the first place. If you don't want to get robbed so much, just don't bring alot of stuff and store them properly where they'll be safe - that's how it works anyway.