Based on the prices on the Flea Market it seems to be a whopping 80/85k pebbles per moonstone.. (I really hope the staff will start taking action, introducing more useable pebblesinks and such, as it has only been getting worse as time goes on)
The only thing you can do is check it daily, sometimes multiple times a day. That's how much it fluctuates. It mainly depends on who's selling them and how aggressive they want to be at out-selling each other. :/
They really don't seem to care about player input in this regard, but unfortunately the only thing that's gonna fix the pebble/ms ratio is gonna be frequent, smaller updates with a consistent dose of new content in each-- from more pattern scrolls to giving people ways to further upgrade their dens or professions. Doesn't need to be a lot at once! Smaller but more reliable updates are far better for a game than a couple big content dumps a year with nothing else to spend on in the interim.
There's still a ton of really simple, nature-inspired irl-critter markings that they could be exploiting to give people even just a couple new things to spend their pebbles on a month, at least for a bit; that wouldn't fix it by itself, but it'd be better than what we got right now.
All that said, the state of the ms/pebble ratio on the FM is grossly distorted by (mostly) just a couple folks, and the only reason it works is because of how small the player base is on top of the wacky economical issues that exist. You gotta get lucky and snag whatever reasonable deals you can in between them trying to see how high they can take it. I know a couple people that are intentionally using the elevated ratio to make $$ on cross-site trading-- I don't remember the details perfectly, since it's not my market, but it's something to the degree of using the inflated ratios to get more bang for their buck on another site when swapping premium currencies.
I believe it was intended for the initial ratio to be 10,000P to 1MS, if we're roughly going off items in the Mole Market and how they're organized by price. It would also make a lot of sense based of similar browser-based pet sims and the usual, neat ratios between their common and premium currencies.
However, things quickly began to spiral, and the ratio skewed to 25,000-30,000P to 1MS. Then it continued to fluctuate and grow. It will only continue to grow so long as people are willing to pay P for MS in the market, regardless of the ratio price. The only way to really get the inflation under control at this point, besides more pebble sinks, would be to avoid buying from anyone who drastically inflates the currencies on the Flea Market.