How much less than the advertised price of a new

car would you expect to pay? If, say a demo Totota was $78,000 how much would you try to get it for? TIA

No idea on the margins for Tototas are, so try 50% off.

For major brands (Ford, Toyota etc.) I would go for 10% off list price.

Have you had a look around to see what others are selling the same for. Being a demo I guess it is already had a price drop from new price.
What would you be happy with if you were selling it?
As kaz would say "Do your homework" (o:

Why do you want a discount? Just want a discount for the sake of a discount? or do you have some reason to justify a discount?

A dealer will be more motivated to move on price if you have other examples you're looking at of similar age, kilometres travelled and condition. Do not walk into a dealer and say "what's your best price", "how much for cash", "what's your discount off that" the dealer will certainly want to know if he moves on price that he's going to get the deal, they will not give you their 'best price' and let you walk out the door.

There is often little room to move on demonstrator vehicles, the dealer will have bought the vehicle into their stock cheaper under an incentive from the manufacturer to register it as a demonstrator vehicle, often conditions apply to dealers seeking a 'demo allowance' on individual units so whilst they may move a little you should not have great expectations, I think even 10% as mentioned previously is far too much to expect!

Be truthful about everything you say to the dealer, if you're going to play their vehicle off against another make sure that other vehicle actually exists and you have viewed it, indicate you're there to do business if they put the right financial deal under your nose then be prepared to place a deposit and commit to a deal.

Maybe this is out of date info, but I used to work on getting accessories and extras rather than discount. The margin on car was far less than that on extras.
That was years ago, maybe still same.

If you look in the search function this question has been asked many many times before and there is a lot of very good advice.
In a nutshell though you are asking the wrong question or questions.
The correct question is
-"What is a fair and reasonable price to pay for this vehicle.?" (or perhaps a bit less because I want a bargain)
On that basis then the line of investigation is pretty easy.