If you look across the spectrum of collecting, lures are really cheap compared to other areas like rare books, watches, art, duck decoys, or even scientific instruments like navigation devices. Once you get out of the thousand dollar area, things get serious real quick. And, you have to know what you are doing. Just because you can afford to invest a few thousand at a time in tackle doesn't mean it is going to be worth squat when you finish. Condition is the first and greatest factor in value and rarity is its twin sister.
Prices go up with rarity, but escalate like a rocket with increased condition.
Completeness of a set, such as the right box with the correct markings and above all the right lure in the box are all important. The trick is to know what is right and what is wrong. If you don't or you don't know someone who does, you are going to get stuck. Since you can't always buy them together, you have to know what goes with what and buy them separately sometimes. Collect catalogs, because that is how the angler decided what to buy and what it would cost to place an order for a given lure.