I spent $37 on a 100-count set of warm white Christmas lights last November, and they covered about nine feet of railing. Not the whole railing. That's 100 bulbs spread over roughly 25 feet of wire, which sounds fine until you realize you want them on the tree and the mantel and the window. Suddenly you're doing math at 9pm with a tape measure on the floor.
So yeah, measure first. Here's what the money actually gets you at different tiers, and what I wish someone had told me before I bought anything.