No. You aren't locked into a price per round when you sign up.
You have a set 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.
We try to update prices as infrequently as we can to keep them stable but lately prices have been going up and we have had to adjust. We anticipate prices will come down in the future as the market corrects.