I hope I am not going to get bashed for plugging my own web site - but as far as I know it is the only utility on the net to answer these questions. Anyway, you should find an answer on how to calculate how profitable a middle is
here . Be sure to click on the link "Go
here to see how to use the line converter" if you have any questions.
In this particular case, the middle is not worth it. Going from 5.5 to 7 is worth about 30 cents according to my converter - and you are being charged 50 cents.
Once again I apologize for plugging my site, which does have advertisements. I felt it was the best way to answer the OP's question though.