Matt baker certainly has a strong following, and his books, in nice shape, do command more today, than "yesterday"... but, they are still very reasonable, for the most part
That is the key... I've yet to see some of the really nice covers offered for sale in any kind of "nice" shape... Even the Sparklecity auction a couple weeks ago that garnered ridiculous prices didn't have the keys in anything higher than F+. The non key nice covers were in grade, I will grant you that, but the others were not and still got huge price tags.
As Dr Love has pointed out, for some Bakers the only high grade copies may be the Church copies -- wherever they may be. I think someone mentioned that even some/many of the Church copies may not be high grade.
I'm not sure whether the print runs on the St John issues were low or collectors had little interest in them until recently. But for whatever reason, the census count is pretty thin on some of them. Perhaps that will change as higher prices bring more copies to market.
Others here are way more knowledgeable than I about this including GAtor but I believe Edgar's supplier did not carry everything, including St John romance owing to poor distribution, forcing him to pick up copies elsewhere - maybe even second hand.
As to GAtor's assessment of prices/values, I think it echoes what romance collectors know - it is really only the best Baker covers that have surged in value, and this does not appear to have dragged up values overall, even on high grade copies, barring the obvious contenders. Sooner or later that semi-mythical high grade CL #25 everyone hopes for will turn up and then we will see some fireworks - but it still wont affect prices on average Baker covers, if there is such a thing! And average copies of average Baker covers now turn up with regularity, where 10 years ago they did not - at least on ebay or in catalogues.