Most expected amount paid should cover maiden with cash falling short (9)

'most expected' is the definition.
(the most common or usual)

'amount paid should cover maiden with cash falling short' is the wordplay.
'amount paid' becomes 'cost' (I've seen this in another clue).
'should cover' indicates putting letters inside.
'maiden with' becomes 'm' (I can't justify this - if you can you should give a lot more credence to this answer).
'cash' becomes 'money' (cash is a type of money).
'falling short' means to remove the last letter.
'money' with its last letter removed is 'mone'.
'm'+'mone'='mmone'
'cost' going around 'mmone' is 'COMMONEST'.