What is the math behind PS+ games?

How much does Sony pay games publishers to put there games on PS+ for free does anyone know?  What incentives other than $ do the publishers get?

I'm not sure they always pay. I remember reading an article and a Indie dev was saying that they didn't mind making their game available on PS+ for free because it really helped promote their up coming release and a lot of people who downloaded it ended up buying the sequal. 

Sony pays them a little with every download probably or something like that. I dont think anyone does anything for free.

Most games that are free on PSN+ have already sold enough and brought lots of money to the game publisher so by the time the game is free on PSN+ the publisher is focused on some nes games released.

What the publisher gets is a good image to the gamers that will work as advertisement to entice gamers to buy some of their games as the gamer was happy to get the older game for free. So it's advertisement for the publisher.

Yea i'm with many of the others, i guess they get a small percentage from sony for each download and it comes out months after release, it's also a good way to get noticed so we buy there next game esp for indie developers, i mean there main purpose is to make games they love and they aren't just in it for the money but for the love of creating something they wanted to create, so giving there game away for free will allow them to be noticed and then some people may buy there next game.