Some writers are beginning to make some decent deals on InkTip... I've heard through the grapevine of a few writers making six-figure deals and you can even read this old interview with Jerrol LeBaron where he sites a deal worth $275K against $575K:
InkTip plays equalizer for screenwriters, detective for producers
However... It is my opinion that these are the exceptions to the rule... The only way to really get people looking at your script on InkTip is to have a great synopsis. By and large, if your story can be instantly compared to something we've already seen, your chances are slim to get anyone to read it.
By the same token, you want your synopsis to actually reflect your screenplay.
Having said that... MOST of the producers on InkTip are in fact low budget producers looking to make a name for themselves and the only way to do that is to find a great screenplay from an unknown and get it cheap. That means a cheap option as well as a cheap sale.
So that's what most of the people trying to sell their script on InkTip are up against but even so... It all comes down to the script and the synopsis. If you have something different that we've not seen before and doesn't immediately sound like some other movie we've already seen, you might have a chance at a sale.
You can see a lot of the deals that have been made through InkTip here:
Total Deals Closed
There aren't any numbers but if you do some searches on Google with some of the information from the deal, you might be able to find some numbers.
filmy
EDIT: I also forgot to mention that there are bottom-feeders trolling through InkTip looking for IDEAS. They can take your idea or several writers' ideas and have one of their own screenwriters write it up. So be forewarned and protect yourself. Make sure your script is both copyrighted and registered with the WGA. Keep an accurate log on everyone who reads the script or exhibits interest and write it down. You never know...