Courtesy of Nattering Naybob.
When a bond is purchased, debt is created and money is lent based on and subject to the influences of three things: perceived risk of entity default; state and local taxes, which corporates are subject to and government issuance’s are not; and perceived systemic risk.
Regarding your question, a “risk free” premium (price) or rate can be observed with the 90 day UST bill. If you want to factor in inflation at the advertised “joke on the public” BS rate, use a TIPS rate or add that BS rate of inflation to the UST bill rate
Remember as mentioned at the outset, there are three influences, so tax premium could be hiding in there. As expected default accounts for a surprisingly small fraction of the premium in corporate rates over treasuries, there could also be a very large spread for systemic risk.
viz. One premium analysis showed that on a particular 10 yr A rated corporate: 18% was for default (leverage considered), 36% for taxes, leaving 46% unexplained. Reading between the lines, probably 80-85% of that 46% was for systemic risk which is non diversifiable. Hope that helps. Out.