Rule of Thumb, PSA vs. SGC Conversion Rate??

[JollyElm]

When these 15% off ebay coupon codes are announced, we all go scrambling to find cards we think would be a bargain. Like everyone else, I grab up cards I want, but always try to keep 'value' in mind to make sure I walk away with cards that would (hopefully easily) sell for more than what I paid for them, or would bring me a good return in a trade. So here's the question. Is there a rule of thumb conversion rate that any of you use when judging what a card NOT graded by PSA should cost?? (Please keep in mind I am asking this question under the basic premise that PSA cards generally sell for higher prices than other TPG graded cards do. Sure there are exceptions, of course, but with regards to post war cards, let's take it as a hard fact for this exercise.)

Based on perceived value alone (in other words, if you're a person who just wants all of your cards in SGC holders, this doesn't pertain to you), say you are looking at the PSA Auction Prices Realized site and see that a PSA 8 card steadily sells for $500. If you see a very similar looking (centering, eye appeal, etc.) SGC or BVG 8 card on ebay (that looks fine and looks to be graded accurately), how much would you be willing to shell out for it? The same $500? Significantly less? A little less? I'm just looking for personal opinions, so feel free to chime in with your own thoughts.