Well, it's a play on stock market where prices go up and down for company stocks similar to the selling prices for the turnips at re-tail. Turnips also have stalks, which is the stem. So that's where that probably came about.
Back to the OP. I only have enough for the silver badge. Most times I don't buy turnips. Whenever I do I hate that I have to sell a whole locker full to get the badges because the total difference of selling and buying turnips matter... It's also too tedious running back and forth just to sell and put back the millions of bells since my re-tail's kinda far