For human readable at sentence level I'd never go less than $5, at the end of the day if you spin it twice you're writing 1000 words, which would be $10 anyway, so take the work load into account.
At word level, for something completely human readable which has been very well spun i'd want like $20. They're going to make thousands of completely readable articles for the price of 4 normal articles.
But people will not pay this much, so do it for the best price you can.