We know you want an exact figure, but it really does depend!
First, age. Older phones are worth less money. So as time goes on, 2017's iPhone 8 and 8 Plus phones will start to lose value.
Then, there's the memory. Selling an iPhone 8 with 256GB under the hood will net you more than a 128GB or 64GB handset.
Lastly, we care about condition. When you get a quote to sell your iPhone 8, we'll ask you to grade the quality of your phone - from Good to Poor then Faulty. The better the condition it's in, the more we'll deposit in your account.