I guess from that perspective maybe the best thing to do is think of it in terms of fun time vrs $$. For example, I spend usually at least one day a week if not two (my weekend) out taking pictures. Normally it's both days but occasionally it will just be one. So if I were to take the amount I spent on lens - $700, and look at it from the perspective that I'm using it bascially 96 days a year, or roughly 768 hours a year, basically I'm spending about a little over a dollar an hour on my hobby and that's if I only keep an use the lens for a year. Not a whole lot of hobbies that are that cheap. Now if I used the lens less or bought a more expensive lens of course the equation would change, but in the long run as often as I use the lens and as much enjoyment as I get out of it I still think it was well worth what I spent.
But really nobody can tell you what the "right" amount of money is to spend on something like this, that's something you kind of have to figure out for yourself based on your needs, your wants and your disposable income.