A Federal "Top Security" Clearance can be worth a lot and shouldn't be looked at in terms of monetary benefit. A Q-clearance (DOE/NRC etc...) which allows you access to TS information can take over a year to receive if you have a lot of foreign travel or contact. Therefore, when an employer is looking at resumes, having a security clearance that is already active could mean that you would be a profitable employee right away versus an employee who would be unproductive while waiting for their clearance. However, if you still want a dollar amount, I was informed by my supervisor that a level L (Secret) clearance is worth around $6,000 while a level Q (Top Secret) is worth around twice that much so $12,000. Obviously this could change, however there is a rough estimate for you.
Here is a salary survey specifically on the value of security clearances
http://www.clearancejobs.com/salary/