I would think not much, since it normally takes a minimum of 25K miles to get a free ticket. So figure 25k miles is worth about $400. But if you sell the miles to somebody dollar for dollar, they might as well just buy a ticket (with less restrictions). So figure even 25k miles is worth only 50-75% of that $400 amount. Did that make any sense?
best place to price and buy/sell miles is dansdeals in the forums. you can find people you will trust, just ask for references and pretend it’s a shidduch. find their friends by looking at their onlysimchas page.