LINESt
 

17.60 37840
42.00 52550
45.80 57250
55.40 65132
336.55 292602
56.50 66408

The 56.5 is the number of acres I want to price and the formula gives me the
correct value according to my hp12c, $66,400.

The problem is that I have to enter the acreage into the formula rather than
referencing a cell.

Can you help?

=SUM(LINEST($O$50:$O$54,$M$50:$M$54)*{56.5,1})

The formula below returns an error message.

=SUM(LINEST($O$50:$O$54,$M$50:$M$54)*{A6,1})

LINESt
 

have you CTRL+SHIFT+ENTER it?

it is an array formula and must be inserted with CTRL+SHIFT+ENTER
instead of using ENTER