There is no simple math equation for how much custom home would cost. Ballpark estimating can be done by square footage, from previous similar properties constructed, but it may have up to 20% error. Average custom homes start from $320 per square foot and can go as high as $500 or even more. Preparing final quotation for custom home requires completed drawings with scope of work and exact finishing materials. Even than the cost errors may be up to 5% due to prices of materials changing on the market.