Instead of using FIND, could you try SEARCH since it takes wildcards?

Maybe something along the lines of:

=SUMPRODUCT(B1:B10)*(-ISERR(SEARCH("MON*",sheet1!A1:A10))+1) or
=SUMPRODUCT(B1:B10)*(-ISERR(SEARCH("MON*",sheet1!A1:A10))+1=1
)

I'm not sure I'm following what the A1:A10 have to do with the answer. The above should sum everything in B1:B10 where you have "MON" with or without any text thereafter at the beginning of cells A1:A10 on Sheet1.

If I didn't interpret the requirement correctly, I'm hoping you can still adapt this use of coercing the result of SEARCH into a 1 or 0 to suit your needs.

Regards,

Shred