I tend to go for the expensive games first when I'm collecting, kinda like a get-it-out-of-the-way kinda thing. I am willing to pay a over a $100 for the hard to find games in my collections, or things I REALLY want. I've already dropped the $$ for Panzer Dragoon Saga, Shining Force III, Shining the Holy Ark and Dragon Force. I dropped a decent amount of cash on my Game Boy Light, because Pikachu seduced me, and for my Castlevania collection I'll have to be willing to shell out some extra cash if I ever want to see a boxed Dracula X, Chi no Rondo (I should con @pceslayer into selling me his), ha or Castlevania Legends...
I'm not sure what my upper limit is, but I'm not sure if I'd ever drop something around $500 for a title. And I'm kinda cheap when it comes to hardware... I totally want a PC Engine CD for when I do get Chi no Rondo, but I'm unsure what a decent price is for the whole set up. Hell, I felt like I splurged overboard when I bought my MGS4 PS3 bundle. haha