cheapest way to go to Hong Kong from Shanghai to make exit and re entry on passport

Yes, it is cheaper than flying to Hong Kong from Shanghai directly. If you just want to entry again and don't want to go to Hong Kong, you can just go through the controls and turn round and come back in again. That mean you need not really to exit. If multiple entries are not useful for you, you can just ask for extension in Shanghai for 1-3 months usually, or perhaps longer, I am not sure. After extension, if you exit and come back again, you need a new visa to enter and the new visa can be handled in Hong Kong either. Hope this is helpful. You can read more at https://www.travelchinaguide.com/embassy/visa/entry.htm