My view:
1) Can you afford the cost without putting any dependants upon your income at a disadvantage.
2) Can you afford to spend a like-amount of money upon any significant others in your life
3) Can you afford to spend the time using the item whilst also providing for options 1 and 2.
In the end if you can afford it and if you're not being the selfish one in the relationship then by heck go for it. The reward is not financial, indeed many things in life do not "make their money back" nor reward you with cash; they reward you in other things. In enjoyment, in what you create, in sparking interest, in getting you through that slog of the day, in generating enjoyment for you and those around you etc...
And yes we all need some moral support when throwing down big money on stuff (esp if we are the lone photographer in the family who don't get why an iphone isn't good enough 
what are my other options, not being a professional photographer? I've been paid a few times for shooting some 1:1 agility sessions with a few border collies, but I'm wondering what other sorts of casual weekend work I might be able to do to help justify having a monster like this. Stock photography might bring in $10-$20 per month, but I can't imagine getting much more than that from it.
