every basic salary is negotiable. considering you are already questioning the amount offered;
there are 2 choices.
1) negotiate salary.
Outcome a) they say no. you say no.
Outcome b) they say no, you persist.
Outcome c) They say yes, you are happy
2) Do not negotiate salary.
Over-riding outcome - if you accept, you are working for less than you want, and you will forever because you are a push-over.
Synopsis:
Option 1:
Outcome a) Nothing ventured, nothing gained. Go elsewhere, you can still put the job offer on your CV. Someone else who didn't negotiate will get the job and be 'underpaid' by the company as long as they work there. You dodged a bullet.
Outcome b) You are demonstrating your value. Have some anecdotal evidence for this (made up or otherwise) or you'll look like an idiot. You are standing out from the crowd and are ambitious. Ambition and talent will get you places, talent alone will get you used. Fact. Balance your arrogance with humility. This is the game.
Outcome c) This doesn't need explaining, other than to say, continue negotiating every bracket in future.
Option 2:
If you aren't willing to negotiate and you're already unsatisfied by the basic salary but accept it anyway, that says a lot about you. Sure, grads with experience are a dime a dozen - but you're not. While every other grad with experience will lap up the offer and roll over and take whatever people dish to them, you won't and you have a sense of self worth. Stick to that, and you'll never be unhappy with yourself.