Occasionally, I get this coupon when entering the Agoda webpage. More often it is a 5%-off discount coupon. You need to activate it and it will show up in your profile as a discount coupon available to you. However, for many Agoda rates you can't apply the coupons.

I definitely get different prices with Agoda as well. I tend to get the lowest prices by finding the hotel in Google map and searching for the rates for my dates - often Agoda provides the cheapest rate and then I follow the link in Google map to the Agoda webpage.

Tip: Always change currency to the local currency of the hotel (you do that in upper right corner on the webpage) rather than let Agoda charge your cc in your currency - the conversion/exchange rates applied by Agoda are quite bad (better to let your cc apply their exchange rates)