You right there is a guided price for the condition of the comic. Unfortunately at some point instead of just adding the cost of slabbing the comic to the price of the comic people felt that they could get 2x-5x what the comic is worth raw. This was just simply because the case would ensure that you were getting exactly what was advertised, its crazy I know.
For example DEATH OF WOLVERINE #1 "QUESADA SKETCH COVER" is priced at $175 9.4 grade. You can find a raw copy for about $499. Or a graded copy 9.6 at $279 and 9.8 up at $1349. So you can really see it's what you can get for them not what the price guide says. I have a great example of that.
For a few years a seller on eBay had part of the final crisis set I wanted to buy all graded at 9.8 6 comics he wanted little over $600 for the incomplete set which was outrageous. So I made a fair offer he declined so I watched, over 3 years this set was on eBay for sale once a year I would send him a fair offer and he still wouldn't budge on it. So yes there is a set price guide, does anyone follow it? I don't think so, it's more a matter of what was the highest price someone else has paid for it previously.