This won't work:

=SUM(T8:U27)-(IF(ISBLANK('Cost Plus Invoice'!O8:O1500)," ",('Cost Plus Invoice'!T8:U1500)))

This formula means,
The sum of T8:T27 (less) the sum of 'Cost Plus Invoice'!T8:U1500 only when cells 'Cost Plus Invoice'!O8:O1500 are empty.

In other words:
If the cell in row "o8" of sheet (Cost plus invoice) is empty, the $ amount in
row T8 will be subtracted from the amount in the cell that has the formula =sum(T8:U27)

Again, I'm sure it's a simple fix but I haven't been able to figure it out.
I tried a round about in my negative into positive # help request. but that seems to be the wrong approach.

I truly appreciate all the help I've been receiving in my quest to finish this rather elaborate spreadsheet.

Thanks, Mark