Copied from:
https://www.ebay.co.uk/help/selling/fees-credits-invoices/fees-private-sellers?id=4822
//// The final value fee is calculated as 12.8% of the total amount of the sale (which includes the item price, postage, taxes and any other applicable fees), plus a fixed charge of 30p per order. ////
So if you sell your item for £500, the normal fees would be 12.8% of this (£64.00) plus 30p = £64.30.
Your offer of 80% off, means you'd pay 20% of this = £12.86
It looks like the £1.00 max fee is a no-brainer :)