I'm always shocked for years how much the digital version can be from the physical even with postage. Anything from £10 to £35 extra. Most games I see on there are £59.99 then a sale is classed as £39.99. Let alone the gold, deluxe super charged editions from £70 and up.
Makes sense from a business perspective; retailers are already offering the titles at lower prices that the majority will pay, so why not charge extra for the 'convenience' of a download? People are clearly happy to pay it, you need only visit the console forum to see people proudly announcing that their entire games library is digital.