"Costed Status" is a Boolean and therefore equal to TRUE() or FALSE(). This means that you can say:
Code:
IF({Costed Status},"Dayend has been done","Dayend has NOT been done")

or, if you find that hard to read:
Code:
IF({Costed Status}=TRUE(),"Dayend has been done","Dayend has NOT been done")

or
Code:
IF(NOT({Costed Status}),"Dayend has NOT been done","Dayend has been done")
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Andrew Bates