Yep. Propane by itself is colourless and odourless. The company is the one that adds the "bitterants" to the propane as a safety feature to stop people from sniffing it and so you know if there's a leak. I'm not sure if the amount is regulated though but yeah if one smells worse than another then it's probably because there's more stuff in it.
PS: If you want really cheap propane, I've heard of adaptors for those large 20lb propane tanks that you attach the coleman canisters to and they'll fill it back up. I'll see if I can find a link for you.
EDIT: Here's a video showing you how to. It sounds like they're called propane couplers or something. I'm also not sure if this is entirely safe (since it's an "uncontrolled fill") but it can't be any less safe than filling GBB mags. I mean we do it on a smaller scale with GBB mags so why not (right?). The coupler is about $20-30 so although it's a big investment it will pay off in time if you use lots of gas.