It seems like grandfathering in the people who already have a 310 is a little unfair to the rest, and I'm one of the people with a 310... I feel like this won't work... They won't nerf your 310s, but I still think you'll have to pay to get the rest to scale, even if you already have a 310...
Also, I feel like it'll be 5k for the 310 part, and as I understand it, flying in the old world doesn't require additional training, you should be able to mount up on your flier the second you log into Cata. Hopefully the underwater mounts are the same deal, or possibly just a 1k gold fee.
I suspect inflation to be fairly low next expansion. BC dailies were 11g, 99s until a few weeks before WotLK came out. and the WotLK ones are 13g, 22s or so. that's not too notable of an inflation.
if i could guess, inflation wise:
super rare vanity mount similar to the trav tundra mammoth and motorcycle: 35,000 gold
daily quest at level 85: 16g 76s
the epic flying skill was already nerfed to 4k at exalted with org/sw, i think that's as far as they'll go
if some new flying skill gets introduced like underwater mounts costing gold, i'd suspect maybe 1500g for it.
The price should be, in my opinion at least 5k, it seems a bit strange to pay 5k for epic flying and then less to upgrade that epic flying (what will it be called, epic-epic flying?). I think 310% should be something you have to work for a little at least, not just a cheap upgrade everyone has it straight away. If they are not adding an extra tier of any rewards above 310% then there will be more than ample time to save up for it from doing quests/dailies.
Underwater mounts are free. You get one in a quest as you leave the first subzone of Vashj'ir. 280% increased speed underwater.