I got a cheap one at Walmart. It was about $15 vs. $30 at #####'s. It's 5 lbs (purple), but not the typical hard surfaced med ball. It's more squishy, filled with sand. But it works fine. The only problem I have with it is when doing some moves in CIA tapes, like putting both hands on it while doing a pushup, I feel like I'll break it.
But I'm glad I got this cheaper one to start with. 5 lbs just isn't heavy enough for me to do strength moves so I think I'll get a good, heavier one. Make sure to get one at a place w/ a good return policy in case it turns out to be too heavy or light.
fyi: med balls are awkward to use the first few times!