It depends on the definition of "net."
Let's say the book has a cover price of $10 (because it makes the math simple).
If it's an ebook, paying what's relatively typical, at 25% of the "net" (with "net" defined as "the amount the publisher gets from the retailer," which is often somewhere between 50% and 70% of the cover price, which I think is where Old Hack's 70% came from), then (again, using easy numbers and splitting the difference between 50% and 70%), if the retailer buys the book from the publisher for $6, then the author gets 25% of the $6, or a total of $1.50. The publisher pays its expenses (editing, cover art, marketing, whatever) and takes its profit out of the remaining $4.50.
For paper books, the industry standard is to pay on the cover price, at closer to 10%, so for the same book priced at $10, with the same discount to the distributor, the publisher would get $6, but the author would get 10% of the cover price ($10), for a total of a dollar. A little less than on the ebook, but consistent with the slightly greater cost incurred by the publisher for printing the paper book.
Finally, there are the scary contracts, where "net" is not defined or is loosely defined as "the costs of printing." If "net" is the amount received by the publisher, it's pretty easy to figure out what that is likely to be, and both the publisher and the author have incentives to maximize how much the publisher receives, because the publisher gets to keep a portion of that profit. Where "net" is undefined, it may include editing, cover art, marketing (all usually the risks assumed by the publisher, not the author), and there's no incentive for the publisher to keep those costs low, because, after all, they get their money first. It's like blockbuster movies that, after creative accounting, produce no profit to be distributed to the investors. Is the publisher going to include some of its general overhead (rent, utilities, stationery, computers) to the cost of printing/publishing your book, since, after all, the book couldn't be published if they didn't have an office and electricity and computers?
It's not so much the percentage numbers that are the key here, but the unpredictability of "net" that's the problem. Even if you had a contract that gave you 90% of the undefined "net," it could be either a windfall (although, of course, any publisher that offered that is likely to go out of business) or -- more likely -- a disaster, when it turns out that there is no "net."
Not giving individual legal advice here, just general information.