I thought that CHDK does not overrides the actual exposure value, but expend the range of available shutter speed, or aperture,
Well, it actually does both.
It changes the exposure values based on what you set in the CHDK menu (of via a CHDK script). And it also extends the range somewhat.
By somewhat, that means much longer maximum exposure times and somewhat faster minimum exposure times. It can't really do anything about extending the aperture settings (f-stop) although getting one additional f-stop smaller has been reported (with a resulting poor image unfortunately). Similarly, you can't slow the ISO value down much but might be able to extend it some.
Note that if you only change one exposure setting ( Tv, Av, Sv or ND filter) then the Canon firmware will use whatever value it was originally going to use for the other values. It will not compensate for your change and you will almost always get underexposd or overexposed images.
You have a good point - should I expect expended range to be shown by Canon display, or CHDK has some other means of showing the actual exposure values?
The Canon values on the display should show what Canon wants to use. You can enable the CHDK On Screen Display (OSD) to see what CHDK will use instead. Also, the EXIF values stored with the image should show what was actually used (i.e. the CHDK overrides) but IIRC this does not always work - especially at extended ranges.
