I don't think it's fair to assume that Fitzwilliam has a good income. I agree that the major problem is certainly his habits of expense, but just being the child of a peer doesn't necessarily give you any money. There would usually be a fortune from the mother, or a grandmother, to settle on the second son, but not always (Elizabeth Darcy's second son, for example, would have no fortune if the Darcys weren't responsible enough to save one for him from their annual income). The great difficulty is that such a fortune would probably not be equal to his habits of expense, but it doesn't seem to have been unusual for peers to set aside relatively small amounts for their children on the assumption that a lady with money would be happy to marry an earl's second son on the chance of becoming a peeress herself someday, and the certainty of good connections meanwhile.
This all raises kind of an interesting question. We tend to see the whole P&P romance from the angle of the people involved, but let's not forget that Darcy and Elizabeth can both fairly be suspected of selfishness by Darcy's great relatives. Let's just say they have two boys and a girl--though as I doubt they will adopt the regimen of separate bedrooms, more is not unlikely. Elizabeth will have nothing to give the second son or to set aside for the daughter (whereas Georgiana's mother probably brought in those thirty thousand pounds) so they'll have to save for them. It seems reasonable to assume that the single Mr. Darcy has not been living up to his ten thousand a year, but they'll probably need to set aside about two thousand a year per younger child to have a fortune of between twenty and forty thousand pounds to give them when they marry, and also give the oldest son a pretty good allowance when he marries, though by that point they'll probably have finished saving up fortunes for the others. That means they'll need to live on about six thousand a year if their family is small. Darcy and Elizabeth will be happy to do that in order to give their children enough to go on with their lives (and let's not forget that would leave their second son with about two thousand a year, and no estate, so poorer than Col. Brandon) but how many people do you know who would live on 60% of their current income in order to provide for their kids? If Darcy and Elizabeth have more kids, the situation could get a lot more challenging.
So all that to say, if Mr. Darcy's second son (assuming a small family) will probably not have more than two thousand a year, and very possibly less, just because his father married for love, we have no idea what Col. Fitzwilliam's situation could be. If the earl has a small family, married money, and is not inclined to "make an oldest son" of his heir, Fitzwilliam could have a fortune of several thousand a year, even if he's used to a lifestyle that would cost ten or twenty. If the earl has a large family and married a comparatively poor woman for love, or wants to leave all his money with the estate to make sure it isn't impoverished (see Donwell Abbey from Emma, perhaps?) Fitzwilliam could honestly have not a penny to his name beyond his Army pay and whatever his father gives him in terms of an allowance. We honestly have no idea--except that he regards himself as dependent on his wealthier cousin in some sense in reference to their travel to Rosings.
This all raises kind of an interesting question. We tend to see the whole P&P romance from the angle of the people involved, but let's not forget that Darcy and Elizabeth can both fairly be suspected of selfishness by Darcy's great relatives. Let's just say they have two boys and a girl--though as I doubt they will adopt the regimen of separate bedrooms, more is not unlikely. Elizabeth will have nothing to give the second son or to set aside for the daughter (whereas Georgiana's mother probably brought in those thirty thousand pounds) so they'll have to save for them. It seems reasonable to assume that the single Mr. Darcy has not been living up to his ten thousand a year, but they'll probably need to set aside about two thousand a year per younger child to have a fortune of between twenty and forty thousand pounds to give them when they marry, and also give the oldest son a pretty good allowance when he marries, though by that point they'll probably have finished saving up fortunes for the others. That means they'll need to live on about six thousand a year if their family is small. Darcy and Elizabeth will be happy to do that in order to give their children enough to go on with their lives (and let's not forget that would leave their second son with about two thousand a year, and no estate, so poorer than Col. Brandon) but how many people do you know who would live on 60% of their current income in order to provide for their kids? If Darcy and Elizabeth have more kids, the situation could get a lot more challenging.
So all that to say, if Mr. Darcy's second son (assuming a small family) will probably not have more than two thousand a year, and very possibly less, just because his father married for love, we have no idea what Col. Fitzwilliam's situation could be. If the earl has a small family, married money, and is not inclined to "make an oldest son" of his heir, Fitzwilliam could have a fortune of several thousand a year, even if he's used to a lifestyle that would cost ten or twenty. If the earl has a large family and married a comparatively poor woman for love, or wants to leave all his money with the estate to make sure it isn't impoverished (see Donwell Abbey from Emma, perhaps?) Fitzwilliam could honestly have not a penny to his name beyond his Army pay and whatever his father gives him in terms of an allowance. We honestly have no idea--except that he regards himself as dependent on his wealthier cousin in some sense in reference to their travel to Rosings.