Random discount => tip situation?

[captZEEbo1]

Sorry about ANOTHER tipping thread, but oh well.

My 2 friends and I go into steak n shake at 4:30am (no managers are working now), we're the only ones there. We order three shakes and the price would normally be around $9.50. The waitress "accidentally" rings it up as 3 junior shakes for a final price of $4.50. How much money do you leave? I gave her $10 total for a $5.50 tip.