Hi Jim,
I do not track SGC, mainly because the sample size is much smaller. I did a sampling several years ago and T206 SGC pricing tracked at about 75% of T206 PSA pricing overall.
Scot
Edited to add:
My T206 price check app is free! It doesn't catch all of the nuances of T206 front/back combos but is reasonably good. Did I mention it is free?
http://www.t206insider.com/store/c1/...r#price-check/