I read somewhere about a survey on this very topic. Their conclusions were that money is not the main motivator. It came in around 7th, after things like job satisfaction, environment, W-L balance, recognition, advancement prospects (which, if you take it to the logical extension, is about more money), responsibility, etc.
So money is not a motivator. But it IS a demotivator. That is to say, if you have enough money it's all good. If you are not being paid enough, you will be dissatisfied and are likely to move on.
In terms of the interview, do mention money (a couple of posts here show that the interviewer has heard this all before, so money has to come into it), but put it in context of your commitments and standard of living, as well as the new challenges and career prospects that you can see stretching you with this firm.
There has to be something that attracts you to the firm anyway, besides money ... on balance they all pay similar for similar grade.