other leading DB Activity Monitoring products

>b) could track unusual amount of data being queried
Say, we can set a threshold of the # of records being queried & if a
table containing sensitive info (eg: NRIC/social security number is
being queried), it will alert us (via email or SIEM)

ideally the cost per DB to implement is less than US$70k

Truly budget has been cut

A low-cost, easy to implement/use (even if it's not that secure) is good enough to satisfy audit requirement:
basically merely to meet audit.  

Just browsed that SANs whitepaper.

So which of the other products  Guardium, Secerno, Sentrigo, Tizor will be under US$35k (yes, budget has
just been cut) & easy to implement/use ?

Since cost is the main concern, I dont think any of the products listed in the SANs whitepaper would fit into your budget.  

For something which will fit into your budget, you would probably need to look at something like the below.  But it still depends on your audit requirement, if this will satisfy what they want.

https://www.manageengine.com/products/eventlog/database-auditing.html?index

https://www.solarwinds.com/log-event-manager-software