Just visited http://base.google.com/base/products.html#price and confirmed GoogleBase/Froogle does not currently support a sale price attribute.
I suspect the Google Froogle mod could be hacked to test for the existence of a sale price for each of your products, and if that condition exists then replace the regular unit price for each of your products with the sale price in an array, before writing the feed file. Or something like that. IANAP.
Woody
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