Will the cost of an unlocked iPhone 4 drop when the iPhone 6 is released?

And by how much?

Yes the price is most likely to drop. The Iphone 5S is coming within a few months and with it all of the current iphones should drop in price. Honestly the amount of money to expect it to drop is completely a guess. I would expect it to drop around 50$ because the Iphone 4 isn't very expensive to begin with. Only time will tell.

As soon as the iPhone 5S gets released, Apple will stop selling the iPhone 4 and 4S so that they can sell their newest iPhone 5S (the short budget one). If I were you I would wait for the budget one as it will be cheap and from what I've seen share the same internals as the iPhone 5 for a lower price. It is all up to you!