No. You aren't locked into a price per round when you sign up. But that goes both ways, you aren't guaranteed a certain price either.
Instead, you set a budget for each caliber. As the price per round changes, the number of rounds that you purchase and are allocated to your inventory changes. If the price is high, you will buy less rounds, if the price is low, you buy more rounds. It is essentially "dollar cost averaging" your ammo purchasing.