Never shortchange yourself - ever. If you have a low current salary, state that you need a significant boost. If the potential employer knows how low yours is, you'll see just what they consider a significant boost then.
That increase will tell you a lot. Are they thinking you're not worth much because of the low salary and give you a small boost? Then they probably won't pay you well no matter what. If they do give you a bit better than you expected of a boost, that means they recognize your talent or indeed want you for the job - a much better outcome.
Consider what your minimum increase is and then when asked, put it on the table. Don't be greedy but be firm; financial firms look for weak negotiators or people that shortchange themselves. You're not worth as much to them if you won't stand up for your needs.