There is no definitive answer to this question as the cost of different shaped pools can vary depending on a number of factors, including the size and location of the pool. However, some shapes are generally cheaper to build than others. For example, rectangular pools tend to be more affordable than those with more intricate designs. Additionally, above-ground pools are usually less expensive than in-ground options. Ultimately, the best way to determine which shape of pool is cheapest for your particular situation is to consult with a professional builder or architect.