[SOLVED] Formula for looking up cost

In B3 and following you have some dates form future (or may be from Mac?) - format this column as full date (with year) to see it).
but if there were appropriate ones - corresponding to row1 dates, you could use standard formula here:

Formula:

=INDEX($L$2:$W$6,MATCH(C2,$K$2:$K$6,0),MATCH(B2,$L$1:$W$1,0))

if you want to focus just on month (year or day does nor matter), then indeed array formula would do:

Formula:

=INDEX($L$2:$W$6,MATCH(C2,$K$2:$K$6,0),MATCH(MONTH(B2),MONTH($L$1:$W$1),0))