I think "prehistoric" pets should be absurdly priced only because they would have to have been on a profile during the V2 switch and most of those pets are loooong gone by now.
I'd also pay a good bit for aged pets because it's not something you can do yourself to one of your pets. Like, if you want a super old pet, you have no other option but to buy a super old pet or wait until one you have becomes super old. Where as with beans, Tags and MPs, you can basically completely revamp a creatu into something new, but it will never be old until it becomes old on its own.