Is a cheap owner a bad owner?
 

I don't have a lot of money. Can I use a plastic tub for housing and towels/newspapers as a substrate? Is it bad for the snake?

Yes, as long as the tub's lid is secured so the snake can't escape, and you can adequately heat it and maintain proper humidity, and the tub provides enough living space for the snake.

A tub like the Hefty 100 qt (40 gal) latchbox you can buy at Lowes is good for most small and juvenile mid-sized snakes because it has a secure latch on all four sides.

I'll 2nd what Melinda is saying. A cheap setup does not a bad keeper make. As long as you aren't cheap on meeting the animals needs, adequate heating (and heat regulation), humidity, food, clean water, then going the cheaper route is a fairly common way for many first time keepers, or thrifty keepers.

That said, do not skimp on the quality of your thermostat to regulate your heat sources. The vet bill for a burned critter will far exceed any savings.

Not at all. As long as you're meeting husbandry requirements, there's no problem looking for cheap DIY alternatives. I do have to agree with Melinda about thermostats and moreover, any heating equipment. That's one place you really want to get quality equipment.