Can i take cinnamon pills, while on metformin, 500mg twice a day?

Yes, but why: You are on one half of the most effective dose of metformin, and could increase at intervals if tolerated. Cinnamon doesn't affect diabetes unless you eat cinnamon rolls which can increase weight and Insulin resistance.

Only harm is cost: Yes, the cinnamon pills will do you no harm. I don't think they will do you any good, either. A healthy diet and exercise is far better for your diabetes than cinnamon.