http://runescape.wikia.com/wiki/Blue_partyhat - exchange price 2,147,483,647
http://runescape.wikia.com/wiki/White_partyhat - exchange price 1,633,035,477
http://runescape.wikia.com/wiki/Red_partyhat - exchange price 1,213,618,860
http://runescape.wikia.com/wiki/Green_partyhat - exchange price 1,094,480,807
http://runescape.wikia.com/wiki/Yellow_partyhat - exchange price 942,856,402
http://runescape.wikia.com/wiki/Purple_partyhat - exchange price 884,445,499
Total exchange value 7,915,920,692
Estimated gold price being .40-.60 / m
You're looking at $3166.3682768 - $4749.5524152 exchange price.
However, that's not all accurate with the street values.
You can't base your payment on the exchange prices, as nobody will sell them. However, good luck buying them.