] ...but I still believe he has an income of more than the £600 pa per the definition Jack posted, especially as the Bennets believe him to have enough money to pay off W.

I certainly agree with you there. We know that Mrs. Gardiner's father left her 5000 pounds, and it's probable that her brother was left a like amount, and possibly even more. Inheritances in the period tended to be weighted heavily in favor of male heirs.

Mrs. Bennet also states that (if her brother had not married and had childern) "I should have had ALL his money." So, my guess is that not only did he start out with more money, he has been very succesful in his business (whatever it is). His children are likely to be raised as ladies and gentlemen