Figure at a cost of millions (4)
I believe the answer is:
form
'figure' is the definition.
('form' can be a synonym of 'figure')
'at a cost of millions' is the wordplay.
'at a cost of' becomes 'for' (what something is bought for).
'millions' becomes 'm' (abbreviation).
'for'+'m'='FORM'
(Other definitions for form that I've seen before include "Class - questionnaire" , "Home of a hare" , "Class in a school, or the shape of something" , "Printed document with blank spaces for information to be inserted" , "Visible shape, class" .)