RORY: OK, the whole concept a free soda is that it’s free, you don’t have to work for it.
DEAN: Sorry you gotta sing for your supper.
To sing for your supper means to provide a service for someone in order to earn a favour from them. The saying is well known from the 18th century English nursery rhyme Little Tommy Tucker, although it was in use before that.
The Annotated Gilmore Girls 
Please write more posts like this about the Gilmore’s! They are very useful. I use this information to learn English and improve my writing (:
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