There's probably a way to do this with set analysis, but the easiest (to me) way is:
sum(if([Old Opp Number] = [Opp Number], [Old Sales price]))
That said, I don't really recommend this approach - why not attach the old sales price to the opp number at the script level? You don't have to actually connect the tables, just bring in the one extra column using a mapping load or a join load.
The suggestion to use if condition is not working.
We have few restrictions not add the old sales price to Table1, because old sales price is being used in other places with other dimensions. If i add old sales price to Table1, it is like having same values in two different tables. If we do not have any other solution we will think to go for this approach.