"So what do I get extra for $150-$450?"

1. The ability to stay dry in a heavy rain.
2. Poles that won't break the second time you use it.
3. Seams that won't rip out the third time you use it.

Having said that, I used a few of those $20 tents back when the budget was a bigger issue than now. After a couple of ugly weekends, I learned that I needed to restrict my trips to fair weather if I wanted to use them. After buying my second cheap tent of the summer and watching it self-destruct, I simply invested in a $40 tarp, which I used happily for about 3 years while I saved up for a good tent (Eureka Timberline, $100 in 1985 dollars, and absolutely worth every dollar.)