IMHO:
Cheapest doesn't means anything if you dunno the source of the product.
They may be selling grade D products even though it's the same, eg. Vit C,
they maybe using the rejected bad oranges to produce so the cost will be down.

Or maybe they'll use synthetic capsules and ingredients compared to organics.
There have been cases in China people died becoz of cheap vitamins.
When they operated the dead remains of the undigested vits dat
causes the death have been found in the stomach of the dead.

Get trusted brands (see Reader's Digest's Trusted brand awards)
Trusted brands will have standard prices though.

ps:
PM me if u wanna get good trusted vitamins
Best of Nature, Best of Science