Roughly three billion yen -- about that amount (7)
I believe the answer is:
thereby
'about that amount' is the definition.
(I've seen this before)
'roughly three billion yen' is the wordplay.
'roughly' is an anagram indicator.
'billion' becomes 'b'.
'yen' becomes 'y'.
'three' anagrammed gives 'there'.
'there'+'b'+'y'='THEREBY'
(Other definitions for thereby that I've seen before include "For that reason" , "about that amount" , "In consequence" , "By that means" , "As a result of that" .)