Probably the only Jamaican Bills you will encounter while in Negril are: $1000 (called a "G") a $500 (called a "Nanny" after the portrait on the bill) and a $100 (called a "C" or "a bill").
A quick "rule of thumb" and not exactly accurate is:
+ a $100 is about a $1 US (actually a dollar and a few cents)
+ a $500 is about $6 US
+ a $1000 is about $12 US
When you see a meal at $600 think $6 (for the $500) and $1 (for the $100) and that will get you close to the amount.
Peace and Guidance