what kind of discount on demo speakers?

Demo speakers are just used speakers.

Some warranties only cover from the date sent to the dealer. Some do cover from the date the customer buys them.
Check to see just how old they really are.
The dealer certainly knows the exact date they bought them.
If they are waffling on the date. be wary.
Make certain the warranty starts the day YOU buy them. not the date the dealer bought them.

Standard used is 50% of new, with varying factors. Are they 'desirable' speakers lots of folks want? Are they cold, nobody wants them?
Then how old. Two years or more is too old and price should drop.
Are they a current model? or a discontinued model?

For five year old speakers where the warranty is from time dealer took delivery, and the speaker is a discontinued model.. 50% tops is a fair price. With damage,, ot would drop to BELOW 50%

For speakers which are hot and folks love them, the current model, and only a year old, plus the warranty starts the day YOU buy them.. 30% off is fine. With a noticable ding.. 35% off.

AND remember:
Since ANYONE can buy brand new current model speakers for 15% to 20% off.. just remember that...

I would be trying to get 20% off the top on new speakers, so I would be thinking more.
I'm sure it depends on the demand for that specific item, but
I've frequently seen much bigger discounts on demos.

I think Elizabeth is spot on. I was able to get 30% off of a demo JL Audio sub and new B&W Speakers as a package. Both items are good sellers these days and the demo sub was only six months old, hardly used. Could I have gotten more off, maybe, but it was a better price than I could find on Agon by far......as a package anyway.

Lots of factors to consider.