If you're going as a superhero for Halloween, you're going to need muscles to complete the look. The time to get started is now (well, probably a few months ago). But you can get pumping by tomorrow and on the cheap with this-- marked down $13 and the lowest price they've ever been on Amazon.
This pair of BalanceFrom all-purpose weights are adjustable up to 40 pounds on each, which is more than enough for me thank you very much. They are made of sturdy vinyl and thus more comfortable to grip, but that does also make them a bit more fragile than iron weights, so don't go throwing them around like in the movies. If you're looking to crank out a few daily sets at home on the cheap, this is about as inexpensive as you'll find a pair of dumbbells.