With PSA, the value of the card also determines the cost of the grade. The more valuable the card, the more expensive the grade. Plus, I kinda don't really want to send my stuff through the mail. I mean, people do it a lot, and it would add a ton to the value of the cards, but damn. In order for PSA to grade my Black Lotus it would cost me $700 minimum because of its declared value. The rest of the Power Nine I have would be in the $85-$200 range each.
BGS is less expensive, I think, but I vaguely remember there being some sort of something that dissuaded me from doing it. I think I could get all 9 of them graded by Beckett for $200 total.