What @Nullo said. If you’re on a low budget, you can load up two backpacks with books or bottles of water or sand, and use that.
Weights are just expensive, I’m afraid, even if it’s just a stupid lump of metal. A dollar a pound sounds like a normal price. (Although I’ve never bought pre-owned ones.)
As materials go, I believe polyethylene is the cheapest and also floor-friendlier than cast iron, so look for that, I suppose. A while ago I got 20kg worth of plates for about €22 that way. (To convert: if they were $1/pound, I would have paid €35,60 for that.)
I think it’s a bit less common than cast iron. Also, the ones I have are rather thick, so you could fit less of them on one bar, so keep in mind what they need to fit on.
Also: are you looking for serious, adjustable weightlifting dumbbells, or just those simple things where the weights and the bar are one lump?