I am also excited about the 13-inch Sony reader, but it won't be available for another year, and I am sure it will be expensive - as Sony needs to recoup years of R&D that went into developing that device.
There are three 10-inch models which have been available for some time, so you should be able to find them used, within your budget: the Kindle DX (which is back on sale by Amazon the way), the Onyx Boox M92 (alias Icarus Excel), and the Pocketbook 9-series (902, 903, 912).
Another option is to choose one of the new 8-inch devices: the Icarus 8 (E800BK), or the Trekstor Pyrus Maxi. You can check if these are within your budget - if not now then in a few months when used ones should appear. Here are some links for these models:
http://icarusreader.com/store/e-read...us8-8-e-reader
http://www.trekstor.co.uk/detail-ebo...oduct/292.html