For a 300 BP character to be on par with a 400 BP character?

Well, it really depends on what you spend that karma on. You'd probably have to gain around 5 points in attributes alone to be at the same level as a 400 BP character in those terms, but you can be very effective if you spend BP wisely. It really depends on how you spend both your BP and karma, so there is no set amount of karma that equals 100 BP.

Andinel is correct. However, giving the 300 BP character 200 karma would probably result in approximate balance. It's on the generous side, but keep in mind that they'll be improving everything from good to better, which is the expensive part (as opposed to making it good).

Don't forget to throw cash at the 300 BP character to help close the gap, based on how much money the other one has made since chargen.