That is still substantially more expensive than flat/symbolic icons, simply on account of it requiring human effort. A substantial number of modern-style icons

That is still substantially more expensive than flat/symbolic icons, simply on account of it requiring human effort. A substantial number of modern-style icons and logos that you see in the wild are obtained by (lightly) retouching the best our of a handful of auto-generated candidates. Since most logos are just "stylish" text now, and most icons are just variations on some letters/geometric shapes, there's a pretty lucrative grey market of tools that pump these out programatically.