Totally depends on the demand for tickets. A tout is very unlikedly to offer much over 10% of the face value.
I would go to the gig and get there early, ask around the touts for a price for a ticket DO NOT say you want to sell one but that you want to buy one.
That will give you an idea of the price. Then when you see someone aproaching a tout , wait until they move away and offer them your ticket at a discount to the 'tout' price you have found.
I did this at an Elton John concert where I found myself with 2 spare tickets , I ended up getting more than the face value for the 2 tickets.