Zero.  I have focused for 35 years on building relationships and a huge network of referrals.

Listings: Because I do advertise my listings on social media, $4.89 on average as a perk to helping my listings broadcast, I get leads from the ads.

My Business:
FREE - yes no cost,  since for my business I'm 100% working by referral.

just time and effort, no paid leads in my life.

My time invested in writing great SEO content and the cost of hosting my website. 

Referrals made and received. 

The large majority of my advertising was on free sites like Active Rain (I did pay for the account to begin with) FB , my own web pages etc.. If I were to guess I'd say it was less than $25.00 each.

A successful closing for a past client. Priceless!

My cost to acquire has always been hard work and developing relationships.  I show people that I am honest, trustworthy, and caring rather than pay for leads.

Because my business is 100% repeat & referral (including agent referrals), the cost for me is my monthly reaches, keeping my websites, marketing sites & IDX running, pop-bys and my quarterly client appreciation events.

Those reaches, benefits of having high-tech at their fingertips (and to market for them when they list their homes) as well as our client appreciation events, all cover the referrals (my CTA) AND my cost of doing business.

I have no idea.

Zero - retirement has some benefits!

I prefer a differant calculation that I beleive to be more revealing.
In all of these actions it becomes easy to bury real costs like blogging, SEO, grip & grin, and those networking events. All are essential to keep the pipeline flowing.
A suggestion may be to calculate ones entire market budget, that means the cost for the CRM, Zdevil, closing gifts, annual calendars and EDD, etc. Divide that by the number of closings. That is the real cost. One may want to include 2000 hours of direct time into that calculation.
For an active agent that could be between $500 to $1500 per closing.
Still a good investment for a $12,000 pay day. Not so good if 50% goes to the broker and a 40% referral fee and 10% to team lead.
Now the question becomes, "what else could one do with $1500 that would deliver better results?"
Like many here, if one's real estate business is over 10 years old, most business comes directly from past clients. CTA = Zero

I'd have to say zero, nada, nothing, zilch unless, of course, you include my time & effort then it's priceless...lol

Virtually zero . . . so much is repeat and referral, the best ones being client referrals . . . 

No clue, none of ours are actually paid for other than a lot of time spent blogging.

Only the cost of my website.   Referrals come from my sphere and repeat clients.

More than half my leads come from my SOI. The rest for a variety of sources. I have no idea what average cost might be.

Depends on which year.  Currently I only pay real estate fees, data base fees and website fees.