In my opinion, the 46, 47, and 49's are the most common dates among the Mobilgas and Mobilgas Special pump plates. I don't think people really look at the dates, I think condition is the biggest factor here. My observation has been that thin white border plates with late 50's dates are the more difficult to find, but previously that hasn't meant a huge price difference with them. (I'm just talking about the common Mobilgas and Special plates) These were made at the tail end of the production of the 5 point shields and as such are probably a bit more scarce.
When I started collecting Mobil 5+ years ago I paid $85 and $125 respectively for nice example Mobilgas and Special plates. I agree the prices paid for the plates in the auctions you mentioned are extremely high and unrealistic, but I think the median prices for Mobil plates have certainly gone up in the last couple years. The low $200s figure seems pretty standard anymore for a nice example of either version, and near the $300 mark is pretty common anymore as well for really nice examples. I'm glad I bought mine when I did!
Always looking for Mobil items I don't already have!