We offer a lot more than Nessus, in terms of doing a deep-dive on web application security. With that said, Nessus does a better job at network security, for example; this is something we're working on.
If I were you I would consider a model where I would be to do a full scan, display only the top X vulnerabilities found, and simply charge more to show the rest of the results.
Another thing I'm curious about: does this work on a pure client-side web application (e.g. my app is just one html page + javascript that loads all the html from templates)? Are you including static urls or somehow tracking "clicks" into a web app? Actually, most of the things I would concerned about in my web applications are things like not validating that I'm correctly doing correct validation in POST-requests. I'd be interested in seeing if you guys are doing that kind of "fuzzing" in that respect (though not sure if there is an automated way to do that safely). Additionally, I'd be curious to see what you detect Nessus/BurpSuite etc doesn't in terms of web application security.
Anyway, neat idea, perhaps I'll check it out a bit more thoroughly and do a comparison.
The best test to see how we differ from Nessus/Burp is to try it yourself! A lot of the vulnerability classes we scan for are very similar, but the ways in which we scan for them are different. We do offer our Standard Plan for a free 30 day trial. Would love to hear what you think :)
If you have any issues, ping us at http://tinfoilsecurity.com/supportchat
Our SQLi and XSS modules in particular are quite a bit heavier than Nessus', but there are other features like page de-duplication that optimize speed as well.
A much more realistic option is Burp Suite, which is $299.
Burp Suite is great for anyone who knows what they're doing; for anyone that isn't already a security guy/gal the UI is near impossible to figure out, and the results aren't particularly actionable. That's much of what we try to fix.
Not trying to be argumentative, just clarifying! :)