Wait, what? I'm confused...aren't DRTs 39 diamonds when not on sale? How is 2 for 79 even remotely a "sale?" O.o
You missed theme of the sale. You are buying 2 items at regular price and getting a "Nice Surprise Orb" gasha as a bonus.
We had that theme for a while now - 95 dia small puri pack and a random chance to get AT charges, 19 dia puri and a chance to get VII or VIII rune, etc.
Discount depends on average value of bonus item to the lost rubies. If value of bonus is worth more, you are getting discount. If rubies worth more, you are paying premium.
In this case, I believe that Nice Surprise Orb usually gives talent charms. If it is something worth a lot to you, then it is a sale. If not, then not.
For me, I am going to be skipping this, because of two reasons - I don't like the bonus value and I don't particularly like the selection of items in pairs.
Did you read, you get 'awesome' orb for 1 dias.
Technically, it should be free and it is not if it costs 1 dia. But then BAS is 15 and 2 of them at 29, so it is all rounding errors of price-ending-in-9 convention.