At negligible cost.
How much? I don't remember having seen the actual cost mentioned
anywhere. And, as John Klensin noticed here, the agreement between the
RFC editor and the FIFA of DOI is secret.
A few seconds of googlage reveals Crossref's price list:
http://www.crossref.org/02publishers/20pub_fees.html
It's $1 per new document, 15c each for the bulk upload of the backlist.
If you divide the budget for producing RFCs by the number of RFCs
published, you'll get on the order of a thousand bucks per RFC.
R's,
John