My wife is a knitter, and I've often discussed the idea of selling Lego instructions with her. In the knitting community, it is quite common to create, publish, and sell patterns online. Prices for patterns are typically in the $5 to $7 range. She estimates it takes approximately 50 hours to write up a pattern for publishing.
One of the bigger knitting sites is www.ravelry.com. An example page of a pattern for sale is http://www.ravelry.com/patterns/library/baby-valentine-sweater . A knitter uploads his/her pattern to Ravelry, and then Ravelry handles the transactions, taking a small percentage of all sales.