Based on information received by REC on June 3, 2019:

For LPFM stations only, Xperi does have a special licensing rate.  The license fee is $5,000.  This can be paid with a $2,000 down payment and then annual payments of $600 per year for 5 years.  You can pay off the licensing fees at any time.  

The LPFM specific license fee allows stations to add HD2, HD3 and HD4 channels with no additional ongoing cost.  Rebroadcast of these multicast channels does not incur any additional costs either.

This of course does not include the equipment necessary to deliver a digital signal, just the royalty to use the HD Radio technology.