Acer's Aspire S3 numbers among the cheapest 13-inch ultrabooks on the market right now, with an asking price of just $876 at Amazon. According to DigiTimes, an even cheaper ultrabook with a larger, 15" display is in the works at Acer.
Quoting "sources with the Taiwan-based supply chain," DigiTimes says this upcoming, king-sized ultrabook will cost $699 and will be produced by Pegatron Technology. The site doesn't quote any specifications or a launch time frame, however. All it says is that Acer is enthused by "satisfactory sales for its 11- to 13-inch ultrabook models," which are pushing it to expand its ultrabook lineup.
The prospect of a thin and fast system with a $699 price tag is appealing enough, but I'm not sure if the ultrabook term would really fit a 15" notebook. At that size, I think bulk might just trump thickness as a chief concern for portability—and there isn't much you can do to make a 15" laptop less bulky, save for perhaps shrinking the bezel a few fractions of an inch.
By the way, Acer might not be the only one with a 15" ultra-thin laptop in the works. Last month, word got around that Apple's next-generation MacBook Air family would include a 15" model. There was talk of some price cuts, too, but those have yet to materialize.