If you cut something there will be a force trying to bend the tool away from the workpiece - and this force can be very high depending on the hardness of the material you machine. The main difference between cheap and expensive machines of similar size is stiffness.But you can spend a lot of money on additional stuff like measuring and clamping devices or dust extractors etc. Except for the router bits that will get dull or break there is usually not much that needs to be replaced or maintained on a regular basis. The additional costs associated with a CNC machine are usually rather low.