Stable Horde has generated ~180 Terapixelsteps of images. Assuming each image is 512x512x30 that is like 22 million images (higher resolutions have an exponential difficulty).
Using the current cost of http://dreamstudio.ai, the Stable Horde has generated for free a value of close to $45000! Using the old http://dreamstudio.ai costs (Stable Horde has been up almost as long), this is closer to $230.000 All this value has been given out voluntarily, with no ads or fine print.
Taking into account the post-processors allowed and the exponential difficulty of higher resolution images (Stable Horde allows up to 3072×2048), these numbers can easily be doubled.
For reference, in its stable horde lifetime, my patreon account has made $500,most of which has gone to infrastructure costs.