Heh, I suppose the lens purchase strategy depends on whether you are a professional, what things you like to shoot at, and what is financially possible. Or if you happen to decide that particular objective would be interesting for your "research" (hey, I do optics in my day job). I checked a 200/1.8 recently, but decided against that since it really wouldn't work with the way I shoot. Too bad for the large aperture though.
Seriously though, since I live in Scandinavia where light is scarce for four months of a year, I do sort of need two sets of lenses. I use fast primes for the winter time, and zooms in the summer, though you won't find me carrying a zoom on vacation.
But I'd recommend not going for the most expensive thing first, but just something on the focal length range that you think could be interesting. For example, Sigma 12-24 is completely sufficient for me in the ultrawide area, while 120-400 does the trick for the 400 mm stuff. Most of the things I take photos of reside between 24 and 200 mm, typically between 24-70 inside and 70-200 outside. 120-400 is more for small animals.
For a vacation, if I think I need a camera with me, I'll probably go with a 20D and 28/1.8 and 85/1.8. That tends to cover all I need, up to night time too.