Hobby-grade RCs use more powerful motors than toy RCs so they also need more durable parts to withstand the greater stress from these faster motors with more torgue and from the hobby-grade nitro engines. The stronger plastic parts and sometimes all-metal parts and the complexity of the construction increase the manufacturing costs so hobby-grade RCs cost more to buy and to maintain.
Mass-market or toy-grade RCs are usually intended for children and beginners who aren't ready or able to invest in the radio controlled vehicles hobby at the intermediate or professional level. They aren't intended to last as long or be upgraded and modified so manufacturing costs are less. They typically use less powerful motors, simplified electronics, and don't require the complex construction needed for vehicles with interchangeable parts and upgradeable components.