I found 2 ways. I'm pretty new here, I'm sure somebody else will find a better way.
Method 1, (and the easier way if you ask me)
Add a collumn next to the collumn with your formula and use this.
Collumn A contains your formula
Collumn B
=IF(ISNA(A1)=TRUE,"$0.00",A1)
Then if you want, you can hide the collumn that contains the main formula.
Method 2 seems far too extreme to me (looks like sand112281 already did this one)
Applying the isna function to your existing formula would make the formula extremely long. But here's how you would do it. Just insert your formula (without the equal sign) in both places where I put a vlookup formula. Something like this :
=IF(ISNA(Insert your formula here)=TRUE,"$0.00",Insert your formula here)
=IF(ISNA(INDEX(INDIRECT("'[2006SalesReport.xls]January06'!I1:i2000"),MATCH($B21,INDIRECT("'[2006SalesReport.xls]January06'!C1:C2000"),0)+2))=TRUE,"$0.00",INDEX(INDIRECT("'[2006SalesReport.xls]January06'!I1:i2000"),MATCH($B21,INDIRECT("'[2006SalesReport.xls]January06'!C1:C2000"),0)+2))