The specific upgrades we paid cash for will probably end up costing 1% of an employee's salary, averaged out over everything, over the lifetime of the lease.
Two points and a necessary footnote:
1) 30% is on the high end. 20%-25% is a normal fee (and in this environment, you can probably bargain your recruiter down to 20%).
2) That cost is what you pay for a recruiter, instead of paying your HR people to do the initial screening. Your recruiter might call up someone knowing that there are ten client companies that would interview someone who graduated in the top 10% of her class at one of the top 10% of schools -- but that only two or three of those interviewers would be a good cultural fit. It's worth it for your recruiter to chase the weird ones (if they're not right for XYZ Co., ABC Ltd. would still love to have them!); the equation is different for in-house HR.
Footnote: I'm a recruiter, who -- sadly -- does not get to work with awesome companies like Fog Creek. Working on it, though.
You think that top software engineering graduates in New York City make $26k per year? Recalibrate! Recalibrate!
I think the number you're looking for is "less than 4%". Remember that the office-construction expenses can be massaged by accountants to reduce taxes on them, and that an employee's (e.g.) $80k salary costs the employer considerably more than $80k, thanks to the employer share of things like Social Security and health insurance. To say nothing of the costs of recruiting a single employee. Note that a headhunter will charge you a cool $30k to recruit an employee with a $100k salary:
http://www.asktheheadhunter.com/basics2.htm
...and who knows what kind of stiff that headhunter is going to try to foist off on you?
The chances that the $4k per employee per year is paying off for Fog Creek are much, much better than you think.