I will second shermae's comment about the 8 cent Monkey. FOr several cultural reasons this stamp remains sought after but its peak value has past. Nevertheless still a valuable stamp and it looks in good condition.
The two cancelled US stamps shown are likely not worth much but it is good to check if it is a variety that is uncommon and therefore may have some value (although nothing compared to your Monkey stamp --- again do check it isn't a forgery. If it isn't one, averages several hundred dollars depending on condition unused see this stampworld site which offers a very broad idea of value and is more on the conservative side -
https://www.stampworld.com/stamps/C...ps/g1617//). The other Chinese stamps shown are of Mao and his writings - after 1966 Cultural Revolution and according to stampworld have a nice premium above their face value - you won't get rich but depending on the stamps ranges from $25 to $100 per stamp, again check these for forgeries -
https://www.stampworld.com/stamps/C...mps/g1013//)Judging from what you have shown it seems your mother was a knowledgeable stamp collector so if you have more stamps like the ones from CHina and better US ones, then the whole collector should be reviewed by a reputable, professional dealer. American Philatelic Society is a good site to start your search for a dealer OR a stamp club that could point you to one.