extracting negative numbers

hi! I want to ask help from you guys,


I used this code to extract the value on the text: "11/09/09 backyard $400"

=IF(ISNUMBER(SEARCH("back",A4)),RIGHT(A4,LEN(A4)-FIND("$",A4))+0,"")


and return "400" but i've encountered a value which is negative (could be
-$400, or ($400); and i got an error of #value, could anyone resolve this problem?



Thanks!