To clarify the question of where the dust is, consider that the image projected by the lens onto the sensor is inverted, left/right as well as up/down. Thus a bird in the upper right of the frame will be projected onto the sensor in the lower left. Inversely, a dust blob on the lower left of the sensor glass will appear in the upper right of the image.
But that's when "looking at" the sensor
from behind. Turn the camera around so that you're looking into the lens mount (but still with the viewfinder hump on the top) and the dust blob will be located in the lower right.
Thus when you're looking at the sensor when cleaning it:
- dust on left of image => left side of sensor
- dust on bottom of image => top of sensor
By the way, it's not actually the sensor you're cleaning: it's the filter glass on top of it. For most intents/purposes the difference is moot, as the filter is extremely expensive to replace (it's a combination of IR-cut filter and anti-alias filter). Depending on the camera
model it can be 1/2 to 2/3rds the 2nd-hand value of the (unmarked) camera.
However, if you do end up with a camera with a damaged filter, you can get it converted to an infrared camera (either to keep or to sell). In that conversion the filter is replaced with different glass. Consider that some people buy new cameras and then remove that filter, and you'll realise that there's a (admittedly-small) market where you could sell your camera for almost new replacement value...
I happen to run one of the few IR conversion operations in the world, so drop me a line if you get to that stage.