Cheeky Monkey! I think you knew you were asking one of those "How long is a piece of string" questions.
But there is an anwser - of sorts and it's not just "It all depends"
I depends on the resolution, colour depth and compression. When you save an image in jpeg, you can specify the resolution, the compression and in some applications the colour depth. Other image formats will give very different results.
Here is a picture of Gangi on the island of Sicily. The original file size was 550KB. This is 64KB. The resolution is 100 dpi and the compression was 8 when saving it as a jpeg file. The colour depth - that is the number of shades of colour that might be used for each pixel was, I think, 24bit. In greyscale, with only 256 shades of grey, the file would be much smaller.
