I hate it when people use the term 'invest' in this sort of context.
You're just spending it. Unless you're a pro you're not going to see a return on investment. 
Ok, so it's not "investing" in that sense, in the same way as you don't really "invest" in a car, you're unlikely to see a return.
But it wasn't my phrase.
I've spent money on lenses, but not made any money from them. Although I did make a small profit selling one on ebay.
The point is that it's worth spending more on your glass than the body, because a lower end body can make good use of good glass and when eventually you want a better body, you'll have the glass to match.
