B1: =ROUND(A1/50,0)*50-0.05
or
B1: =MROUND(A1,50)-0.05
(MROUND is from the Analysis Toolpak)
Note: the lowest price that works for is $25.00. Under that, the formula
calculates $-0.05. If you want the minimum price to be $49.95, try one of
these:
B1: =ROUND(MAX(A1,25)/50,0)*50-0.05
or
B1: =MROUND(MAX(A1,25),50)-0.05