Try these array formulas**:

A1:A10 = range of values
C1 = new value

For the next lower:

=MAX(IF(A1:A10<C1,A1:A10))

For the next higher:

=MIN(IF(A1:A10C1,A1:A10))

Note that if there isn't a next lower or next higher you'll get a result of
0.