Airlines and last minute flight purchase question

[Abble]

What are the odds of turning up at an airport to go somewhere and just book flight somewhere there and then ?
ie. hope for a plane to have at least one spare seat ? would airlines then let you pay and walk on ? are you going to be charged a packet ?

[Sausalito Bay]

i. Good, about 6/4
ii. They usually do
iii. Yes, most definitely
iv. Very probably