This is even more of a joke. You said most directories use Wordpress and now you are suggesting a simple WP directory will cost you 5K!
Do you have any idea at all how a CMS actually works?
I don't want to appear rude or childish but setting up a wordpress directory will cost you nothing at all.
Only if the entrepreneur also happens to be a web designer - most ain't, but most designers I know won't do squat for free - they charge for everything. I'm quite sure you charge for your design services.
My rough site design costs over the years:
Copywriter biz site - £50
1st Basic directory - £100ish (and I do mean basic.....)
2nd directory - £25'000 (including 3 re-designs)
Blog - £200
Forum - £200ish for license
Job Board - £2'500
--------------------
Directory turnover - £18'000 2nd year of trading. We spend roughly £3k per site, each site brings in at least 12k per annum, and traffic is easy to transfer to each site, site gains fast traffic, gets bookmarked and repeat traffic sets in. Just rinse and repeat methods, but quality and no half measures, and no rushing to build the greatest site stuff either. just planning and gradual build-up of content and traffic.
There was a blog platform, and was run by I think 3 friends, who devoted much time to it, they struggled and nearly had to shut it down, it was eventually bought by Google. Another directory sold for 2 million to Yell.com (I forget the name now) - and a Travel site began life as a bedroom project by 2 mates - called HolidayWatchdog.com - acquired by Expedia.
Ofcourse, there could be some luck involved eg: right time, right model - but ultimately its the idea that makes a website work, the cash just accelerates the in-bound traffic flow. Just one of my websites got 1600 sign-ups in 12 months online, just by word of mouth, no ads, no press, a few hundred links in the right places so b2b's could find us.