I "guesstimate" the grey sheet prices by taking approx. 80% of the numismatics list price as a guide.
Not perfect, but better than paying for the grey sheet, IMO! I got the complimentary copy of it and just don't think it is worth a subscription for most collectors.
Heritage lists a wholesale price as well as several of the major retail lists - if you are signed in. IMO, that and the past auction lists there are a great source to approximate off of!
I use Heritage for this far more than anything else. Between it and
ebay past auction results I think it is the best way to get a good idea what the current FMV really is.
JMHO