It is important to choose the mass* of the robot sooner rather than later as it will have significant ramifications on the total budget of the robot. As a rule of thumb, the weight of a robot can be divided into electronics, mechanics and battery. Because each mobile robotic design is unique, an estimate of 30% for batteries, 50% for mechanical frame and hardware and 20% for electronics can be used (these numbers will undoubtedly vary depending on the type of robot). RobotShop has divided mobile robots into three rough weight categories:
Small
If you are new to robotics and are looking to gain knowledge and experience without breaking the bank, this is the category for you. Small robots are generally considered to be less than 3kg (6.6lbs). Although some of these robots can be expensive because of specific functionality and requirements, these robots generally cost between $200 and $1000. Small sized robots are not necessarily restricted to indoor use and often have the benefit of being easily portable. An example of a small robot is the POB Technology BOT.
Medium
Medium sized robots are generally considered to be between 3 and 20kg (6.6 to 44lbs). Because of their size, medium robots are capable of carrying increased payload, the cost of these robots can vary tremendously, but usually average $1,000 to $10,000. An example of a medium sized robot is the Coroware Development Platform.
Large
Large sized robots are 20kg (44lbs) or more. They require larger and more costly motors and motor controllers. These robots can include industrial robots, de-mining and surveillance and even small electric vehicles. These robots require very robust frames and often include redundant systems. Prototypes cost several thousand to several million dollars (for research or military prototypes).
» » How Heavy Should I Make My Mobile Robot