Looks like the prices are surprisingly similar, though cinnamon is, if anything, more expensive. Additionally, I'd imagine that if sodium benzoate is the active ingredient, you'd need less of it to get the job done.
I'm not sure if you can assume this. I'm not sure myself, but the path that sodium benzoate finds its way into the bloodstream (or wherever it needs to be) is a bit more complicated than just "eat this pure sodium benzoate".
Apparently, cinnamon can be used as a vehicle for moving sodium benzoate to wherever it needs to be.
Just in case anyone is thinking of purchasing through Alibaba:
Alibaba is notorious for scammers. I know two startups that have lost around $20,000 each on material suppliers listed there who send something cheaper (usually completely different) than what has been requested upon payment. It's usually the orders that are around $1000 that Chinese chemical/raw materials supply companies selectively scam on.
Apparently, cinnamon can be used as a vehicle for moving sodium benzoate to wherever it needs to be.