To me buying cannabis is like buying any other product. Schwag is sorta like the store brand; sometimes there is no difference or very little difference, although other times it's the most god awful thing you've ever tasted. It's always a risk, but if it's cannabis you should still get high if you smoke it.
Several times I've gotten some awesome mids; however last night I had the shittiest weed I've ever smoked. But come on even though it may be economically better you can't always go cheap...you gotta splurge sometimes. Its worth it and when you get a good strain no schwag can compare to it.