Mountain bikes have wider chain stays than road bikes so the cranks need to be further apart to prevent them (or the cyclist's heel) from banging into the frame. The distance between the pedal attachment points on the crank arms is referred to as the Q factor. Typical Q factor for road cranks is ~150mm; for mountain cranks it is ~175mm. This means a mountain crank is likely to be 10-15mm further from the frame than a road crank.
Small differences in the crank symmetry aren't that noticeable, but I suspect that 15mm might be. Also worth noting that many people believe that narrower cranks are more comfortable and efficient, which is why manufacturers produce their road cranks with a lower Q factor.
If you want to replace your cranks on the cheep, I would try to find a second hard pair compatible with your existing chain rings and bottom bracket. Do swap the chain rings, even if the replacement crank has rings, because differences in the wear between rings and chain can accelerate wear and cause chain slippage.