What software for security testing the optimal ratio of price/performance?

Nessus, HP Webinspect, IBM Rational AppScan.
I really like the last one.

Hacker Bob. Recommend. Artificial intelligence has not yet drawn, and only one base exploits are not enough.

Depends on what security. For example:
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skipfish — A fully automated, active web application security reconnaissance tool code.google.com/p/skipfish/

From free, in addition to the aforementioned there is still a good skipfish Websecurify and w3af.

Suggest manually. If you need help, write.