They can't respond once the ability has begun to resolve and they see what the result of the coin flip is
To determine the total cost of a spell, start with the mana cost or alternative cost you're paying, add any cost increases (such as the commander tax), then apply any cost reductions (such as that of Dargo's first ability)
Anything you want to accomplish with Rionya can be attempted with Delina, Wild Mage too, but its much harder to pull off, given she makes a tapped and attacking creature, and given that creature will be exiled at the end of combat which will happen as you move to the next combat phase
How much ramp and card draw should I include
This same concept is applied to the list cards or other "bonus sheet" style mini-sets
Evaluate The Card's Condition The condition of your cards can also significantly impact Magic: The Gathering card value