OK, so I know there lots of factors in this, with lots of different answers. So let's narrow it down a bit.
I recently read a blog that was talking about literary agents who were interested in taking on self-published authors. Sounds great so far. Then it seems they have a criteria of your having sold at least 5K books in the previous year. Hmm, a bit more difficult. I'm guessing if you're selling e books at 99c they're going to want to see considerably higher sales than that before they take an interest in you.
So, assuming I can sell that many books (sounds optimistic but with the e book market growing you never know) would it be worth my while going down that route?
Let's think about this from different points of view.
Let's assume they mean 5K sales at a price that gives you a royalty of at least £1 per book, ie an annual income of £5K. That allows for a slightly higher return at the magic price of $2.99 on 70% royalty but nets down allowing for expenses. That's still a higher royalty per sale than you'd get with most publishers. Let's assume your book is currently selling well. So, if you meet their sales criteria, to make it financially worthwhile, given that you'd probably have the book off the market for at least a year, and thinking about projected increase in e market and therefore your expected self-published income over the next 3 years etc etc, you'd realistically be wanting an advance of at least £20K.
This is without factoring in all the extra work you might be required to do on rewrites etc when you could be writing your next book which will earn you another £5k per year. Plus, you might not like how you are required to rework it. Not such a big deal when it was an embryonic ms maybe but more like invasive surgery further down the line.
So, what's in it for the agent/publisher?
Money. You've found a way to make it, you've already done all the hard work, so now they want some of it.
The book? Probably irrelevant, though if you've got more on the way you can give them more money.
Remind me again what's in it for the author.
Fame, fortune, national press reviews, book displayed in bookshops, reaching a bigger audience, being 'properly' published.
So, assuming you meet the criteria that a mainstream agent/publisher would be interested in signing you, tell me what your criteria would be to consider signing with them.