If you follow the pricing formula as described in most of the Online Labs, your wholesale, or bulk, pricing should be about half of your retail. Now, obviously this doesn't translate in every shop. It depends on your profit margin. I personally think a discount is only a discount if it's 20% or more. If you can do more than that without stealing from yourself, I'm sure the customer would appreciate it! If not, I think 20% is the perfect discount amount, especially on bulk orders that aren't priced at wholesale.
Sorry for rambling, but I hope I could help!
Stephanie