We can save 'em for those that really deserve it. Like the French.

Deal.
Essentially, anything I don't already have. Of the pricey titles ($100+), methinks the only ones I don't have yet are Dynastic Hero, Super Air Zonk, and Magical Chase.
Sounds like our goals are the same. This does bring up the question of what games you believe are $100+ titles. Here are the titles I'm missing (no, I don't care about Bikini Girls or whatever, nor different colored Champions Forever-at least not for now

):
Beyond Shadowgate
Bonk 3
Cotton
Legend Of Hero Tonma
Magical Chase
Might And Magic 3
Soldier Blade
Super Air Zonk
Terraforming
Note that pretty much every one of those titles are generally considered expensive, I started with the cheap games. I don't think I'd be upset to pay more than $100 for any of those, though I'm harboring hope that I'll find a loose Bonk 3 for less. Several of those I full expect to pay more than $200 for. Magical Chase is the only one on the list I'd pay $500 or more for.
'Til the cows come home.
I think this is the major area we disagree on, but maybe I should give a bit of context. First, prices are dynamic and with few exceptions they don't tend to get cheaper. Even in the 2 years I've been trying to collect, prices have jumped quite a bit (perhaps due to my own actions). However, it seems safe to guess that the longer I wait, the more I'm likely to pay for a given game. Moreover, I do have a decent job right now and due to the unfortunate death of my grandmother back in March, I've got some extra savings.
To put some time frame on it, I'd like to finish off my collection sometime in the next couple years. I certainly don't want to wait indefinitely to complete my collection. I fully expect Magical Chase to be the last title I buy and I don't expect it to be cheap.
For most games, I consider anything at the low end of eBay prices to be fair, but that doesn't work too well for less common games, like the above three. Off hand I couldn't say how much I'd pay for one of those today, but it certainly wouldn't be as much as the recent high BIN prices.
Cop out!

That's cool, I'm sure it depends on any number of environmentally dependent variables.