Right, the rationale between charging more for a higher declared value is that the TPG's guarantee the authenticity of the card and the grade. (I know folks will argue about the WWG Dimaggio since it looks like PSA isn't backing off the 7, but let's not get into that here.) Therefore, if the TPG is incorrect, they have to pay for the difference in the value of the card between the correct and incorrect grades or entire value if the card were not authentic. You can think of this as a kind of "insurance," so if the TPG has a higher liability on the card, it would only be fair that they would charge more for that. For example, I believe SGC may have had to pay out due to incorrectly authenticating a few D350-3 cards a couple of years ago including a Ruth (
Link) which could have cost them tens of thousands of dollars. (As a result of this, I believe SGC is now no longer putting many stamped back variations on the flip.) Therefore, it seems justified to me that a TPG take higher fees for a 10K card than a $10 card due to these liability issues. In addition for higher dollar cards, I believe that the TPG does spend more time with the card. I remember on the PSA forum, in one thread (
Link), SGC did not certify the grade for the card because it would be very valuable with that grade, so they wanted another grader to confirm the grade on that card before they allowed the grade to pop. And that grader was on vacation for that week, so the submitter would need to wait a little longer to receive the their card back even though they paid for a 2 day service level. So it is both greater liability and greater attention to the more valuable cards that factors into the higher service fees.
I just wanted to add that Beckett actually does not charge a different service fee for the declared value of the card. They only charge different fees based upon turnaround time. Only PSA and SGC charge different fees based upon declared values. So you can say it can be a business decision by the TPG also.