The fact that you put up this question might show that once you agree to the offer you will always ask yourself whether you could have earned more at another place. So only agree to the offer if this will not bother you for the rest of your time being there. Plus -as already mentioned here by others- what is your upside potential? Are you going to learn something new? If the chances are low that you might learn something new and you can't get your head around earning less than average, I believe you already know the answer.