You can avoid performing the complex calculation more than once:

DECLARE

@a INTEGER = NULL,

@b-2 INTEGER = -351,

@C INTEGER = 45,

@D INTEGER = 999;

SELECT

Calculation.result * ((SIGN(Calculation.result) + 1) / 2)

FROM

(

SELECT TOP (1)

result = COALESCE(@A, @b-2, @C, @D)

ORDER BY result

) AS Calculation;