All airlines require you to purchase a minimum air fare in order to upgrade to Business Class with your air miles. This minimum fare is usually much higher than the cheapest Economy Class fare or seat sale specials you might have initially been offered. This means that you can easily pay three or four times more for your ticket than you expected in order to redeem these miles.
However, once you have purchased this ticket, your upgrade is not guaranteed as airlines only allocate a certain number of seats per flight for upgrading.
We strongly recommend that if you do not have enough miles for a full ticket, you don't use your air miles for a Business Class upgrade. Your flight could end up costing you a lot more than necessary. Should you be happy to incur these costs however, our Flights Experts can help you purchase upgradable tickets.