I don't recall seeing anyting specific for SPG, but apparently Hilton does not see this as being that secret. Most of my eFolios online for reward stays not only includes my incidentals, but also a page showing the HH billing. The rates ranged from $10 to $30/night plus taxes.

My guess is the hotels have some involved formula that includes room rate and occupancy. A few times I've been billed the transfer rate. I've offered to accept the billing IF they would return my points. Funny thing is NONE of the properties accepted!