My hourly rate is $400.00 (US) per hour, rounded up to the nearest quarter hour. If you get a monthly retainer agreement, you will get a substantial discount from this rate.
Security audits very much depend on how much PHP code there is for me to scan. If you send me the theme or plugin that you want scanned, I can give you a flat rate or at least estimate how many hours it will take.
If you know what you want, and the scope of your project is strictly defined, I might be able to quote you a flat rate for the job.
I’m also open to discounted monthly retainer agreements (essentially prepaid hours to use within that month). You’ll get a discount off of my hourly rate and I’ll make sure I’m available to provide those hours within the month. This is what you want if you constantly need WordPress work done, and need someone to answer the phone when there’s a problem.