Guys, please help me in correcting the formula. Below is the data

I have this formula "Results = IF PRICE 1 = 0 THEN 0 ELSEIF
IS(NULL) ([PRICE1]) THEN "ERROR" ELSE PRICE1 - PRICE2 ENDIF
Pease advise how to fix this if Error is states "the formula in Price1 resulted to string format. Use to number..."
Thanks,
Kamen