Note: it is now called Adobe Reader. Adobe Acrobat Reader was the name 10 years ago.
Yes, there really is a cost. You apply to Adobe, and they examine your case and justification. They might approve it, and equally, they may reject it. Then they will negotiate a price with you, perhaps with a one-off cost and perhaps with an annual fee, or both. The fee will depend on volumes and type of plug-in. The most expensive are DRM plug-ins, which seem to demand five figures, recurring.
Finally, if approved and paid for, they issue you with a key and you use it to transform your Acrobat plug-in into one that can load in Reader (it remains loadable in Acrobat). This will be valid for a specific release of Reader only.