It sounds like your pet may be 'Obstipated' instead of just Constipated.... When you say colonoscopy, is the veterinarian recommending anesthesia with manual removal of the feces? If this is so, then we may have some form of obstipation occurring.
Once the colon is emptied out - then we need to figure out a longer term issue which may involve food changing. It may involve chronic use of laxatives. It may also require looking for an underlying problem that is causing dehydration such as kidney disease, etc....
If the procedure is too expensive, then I have have some cases respond to repetititve awake enemas. It may take longer to be able to de-obstipate (if that is what is going on).
I hope that helps.