It can bounce around a bit, but when taking all the different items for sale on the FM in both MS and pebbles into consideration, I see it mainly averages around 1MS to 20k pebbles.
I don't go by the selling pebbles for MS and vice versa because that ranges to widely and randomly.
Thanks @AsteriaDust trying to get the 225MS needed for Leopard and Merle patterns I have planned. At 54MS currently and 1.47M in pebbles so was hoping I could get quiet a bit of MS for that much