Your camera probably has two settings for jpeg. One is the size of the image in pixels, and the other is the jpeg quality. If both are set to maximum, the file size will depend on the amount of detail in the image. (JohnG mentions noise, which the compression software interprets as detail) A picture taken with your lens cap on will give you the least amount of detail (none), and you can take this as the minimum file size for the selected quality/image size settings you have chosen. To see how this varies, try choosing lower quality and take the same shot of your lens cap.
The jpeg compression at highest quality varies from camera to camera. My 5MP Minolta 7hi has larger jpeg file sizes than 6MP Pentax Dslr. One reason I choose to shoot mostly RAW. ( I get to control the detail level and how to save the pic - OK, I'm a control freak)