High torque motors are virtually just a regular, internally commutated brushless DC Motors mounted 
with extra gear mechanism at the top, to either raise the torque or to reduce the RPM (Revolutions each and every minute). These DC motors are peaceful effective and can be used in several hobbyist and industrial application.
Reducing the RPM whilst maintaining high torque might require adding up more technical gear mechanism hence the price of these High torque DC motors mainly depends upon the RPM, torque and quality of the motors.The engine shaft from the base engine though a transmission connects the tiny gear box fixed at its front portion, which in turn generates the required torque while keeping the RPM low, the whole gears and gear heads are doped with thick viscous grease lubricant.
These High torque Motors come on different RPM's such as, 5 RPM, 10 RPM, 100 RPM, 300 RPM, 1000 RPM and so on. The cost of these geared motors is based on the RPM, usually lower RPM costs a lot more than those of higher RPM.
Applications
High torque and low rpm DC motors are well suited for robotic and hobby applications, thought the size of the electric motor is a matter of concern since it is quiet large in size due to additional gear box but nonetheless it would save the trouble and cost of adding exterior gears to the assembly of Drive Chain robots where simply the regular DC motors wouldn't prosper. The High Torque, Low RPM DC motors comes with inbuilt gearbox mechanism providing required torque for your robotic application. Hence these motors would be the the majority of favorable choice for the hobbyist robotic tasks and tutorials posted here.