Another point against breed change prices, common breeds are 100kp. When you convert that, they cost about 5-7MS at 15kp-20kp per MS. Meanwhile, a Zerda converted to pebbles is 2,250,000 to 3,000,000. That is a HUGE jump for uncommon and rare breeds.
Also, while the purchase price of MS and Gems is about the same, you have more power with MS. The people who are buying MS are basically controlling the economy right now. I would also like to point out that when FR was freshly launched, you could buy 1 gem for like 100 treasure. There was less to buy for Gems as well and a lot of desirable things cost treasure. Once you catch up recipes and pebble familiars here, there isn't much to buy aside from den space. (And you can get just about every pebble familiar with Arena Tickets, so you don't HAVE to spend pebbles on that.)