If we want DOIs maybe DataCite … https://www.datacite.org … would be a better provider than CrossRef. In any case I would imagine the subscription to assign DOIs would not cost more than 500 euros/year.
A cheaper option closer and a more open approach is EPIC (50$/year). Through EPIC … http://www.pidconsortium.eu/ … we could assign persistent identifiers (PIDs) using the same resolution service used by DOI (Handle system) however the persistent identifiers will not branded DOI.
We should clarify what we want a PID for. The easiest would be to have a PID for each training material and course entry (web page). We could make it more complicated and have a PID per document submitted in each material and course page.