From a purely economic standpoint, the ATtiny85 makes sense.
eh? Assuming simple blinking it the desired functionality, a 555 is $0.41, an ATtiny85 is $1.18 (qty 1, digikey).
The 555 needs a few passives (less than $0.77 worth, I'd wager) and requires more assembly work (which could be viewed as an advantage, for a kit).
-j