Like anything in the world, you get what you pay for.
And I mean that honestly.
Consider this, a reputable brand mid range amp and 10" sub, with a box will run at least $500 assembled.
What do you think your going to get for $100?
I used to work for an auto parts store that sold similar package deals to the one your looking at, and I would have people trying to return them all the time after they blew them out after a week.