I'm a little bit of both. I go for my nostalgia/high-want games first, irrespective of price. After that, I tend to pick up cheaper games if I'm just trying to pad my collection for the sake of padding my collection. My only full US set is the BLISTERINGLY huge 14-title Virtual Boy set

But with that, Jack Bros was
definitely my last acquisition, and Waterworld was 2nd to last.
However, the next full US set I have my eye on (absently, lazily) is Virtual Boy, and I'm currently eyeing up a copy of Web of Fire that someone's selling on a forum I'm on. I'm unlikely to buy it only because I'm trying pretty hard to pay down the last of my personal debt from house renovation (only about 2k... should have it paid off by tax return if not earlier), but I definitely was eyeing it up.