In most settings, nanotechnology is not some cheap technology that can be picked up off the street. While it is feasible that some campaign settings could have nanotechnology so common that any character could go to the local nanotech clinic and receive the latest nanite injection, it is more likely that obtaining nanotechnology for personal use is more difficult than simply dropping down the cash. Gamemasters might limit the availability of nanotechnology by not only making nanocolonies expensive to purchase, but also requiring the character to obtain permission or a permit.
Conversely, the Gamemaster may wish to dole out nanocolonies as a reward for completing a task, or might determine that a nanite injection would be standard procedure for a particular organization.