he's not the one making the trade, but a normal aerodactyl is indeed worth less than a dratini, and a mega aero is worth considerably more than both. charizard mega stones were once available in town lootballs, so are much more abundant than aerodactyl stones (which was available in one event area, but it was well hidden and very few people actually managed to pick it up). mega aero is not a "common mega" by any standard.