mas mobarato actually kung mag GDDR5 sila... kay one of the reasons ngano mahal ang 280 and 260 are its memory bandwidth... 448bit for the 260, and 512bit for the 280... mas pricey actually ni nga strategy, compared to using GDDR5. here's the source.
AnandTech: The Radeon HD 4850 & 4870: AMD Wins at $199 and $299