The 1GB card's GDDR4 memory is apparently clocked at either 1100MHz or 1050MHz (the listing is contradictory), so peak memory bandwidth should add up to either 140.8GB/s or 134.4GB/s. That's an increase of 27.3% to 33% from the 512MB model, whose memory runs at 825MHz and yields peak bandwidth of 105.6GB/s. Core clock speed has also gone up from 740MHz on the 512MB card to 825MHz on the 1GB model, raising peak floating point power 11% from 475 GFLOPS to 528 GFLOPS.
As for pricing, the 1GB card is listed for $649.99 CAD. The same site sells Diamond's Radeon HD 2900 XT 512MB for $464.95 CAD, suggesting that the 1GB model has a 40% price premium. Diamond's 2900 XT 512MB costs $419.99 on Newegg, so applying the same premium there would result in a $587.99 USD price tag for the 1GB card.