This has been discussed many times here, but you will always have to set your own worth in pricing a service like music.
The local market will dictate what it's worth to them, but you have to be happy with the price or you won't put your heart into it. There are many full time performers on this forum and our rates probably mirror each other in some ways, but in certain markets - the money will vary significantly.

As a part timer - you should have no trouble selling a duo for $150. There are rooms that will pay the same whether it's 1 person or 4, so keep in mind the service you are providing. If you are there for 4 hours and the bar takes in an extra $500-$800 because of the entertainment - it's worth it to them to shell out $150 for you. Less than that, and music becomes a liability.

One thing I suggest is that you set a fee and stick with it - ragardless of clients. I never bargain with my rates in the market I work (mostly private parties) so no one gets quoted a different price. I have a set hourly rate for weekdays and one for Saturdays, and they either pay the ticket, or they book a lesser act.( )

I know I tap danced around the numbers here, but you get the idea ...... there will surely be more comentary to follow. This place is loaded with opinions.