how to 'paste' formula in multiple lines with keeping part of formula constant

thanks in advance.
i have essentially a receipt spreadsheet (E2007) where i multiply cost [different prices by column](E3, etc) by quantity [differnent numbers by column](E4, etc) and sums total into O4. i am breaking each day down by customer, so would like to copy my formula, but the cost row will be constant. when i copy/paste into subsequent rows, the formula updates each function line item.
ie what was the value of O21:
=SUM(F21*F51, G21*G51, H21*H51, I21* I51,J21*J51,K21*K51, L21*L51,M21*M51)
becomes for row O22:
=SUM(F22*F52, G22*G52, H22*H52, I22* I52,J22*J52,K22*K52, L22*L52,M22*M52)
but what i want for row O22 thru indefinate rows, is:
=SUM(F21*F52, G21*G52, H21*H52, I21* I52,J21*J52,K21*K52, L21*L52,M21*M52)
without having to retype at each row.
i am open to an easier formula, than what i currently have, for the total cost too!
i hope thats all clear!

[I]well, never being so silly as to refuse help...here's a sample. you all are great for replying so quickly! your suggestions are so helpful, feel free to keep 'em coming![I]