You're rap protégé Chief Keef ... you're making buckets of money ... and all you want is some delicious McDonald's grub ... easy right? THINK AGAIN.

TMZ obtained hilarious video -- courtesy of hangwith.com -- of Chief and a buddy driving through a McD's this past week in Illinois ... after ordering almost $50 worth of food. Food you can ALMOST taste.

Problem is ... when Chief goes to pay he runs into a major -- and pretty rare -- monetary problem. It's definitely not what you think.

Unless you're thinking of making it rain with hundos. Then it's exactly what you think.