When working on a double-sided marketplace how do you work out cost of customer acquisition?

I think you're looking at this the wrong way. Your customer acquisition cost is not something you should benchmark against other businesses. Without knowing more, like your short and long-term goals, it's impossible to answer. Two companies with similar business models may have different answers to this question. A venture-backed startup trying to keep up with aggressive revenue goals may be able to stomach an astronomical CAC. A bootstrapped startup that is not seeking venture money may aim for slower growth and much lower CAC. I suggest setting up a call with a marketing or finance expert to determine what CAC is appropriate for your company and how to get there.

Focus on the more difficult side of the marketplace. For instance, if you think it'll be easier to get suppliers, then focus first on getting buyers - always be working on your toughest problem (aka your biggest risk). You'll find some great blogging on Marketplace and Platform topics here http://platformed.info (read the ebook too!)

To answer this, you need to have a working model for your customer LTV (Life Time Value). Most startups can't accurately estimate their LTV, so if you don't have good enough data, then building your CAC based on assumed LTV numbers can be fatal. In this case, it's better to evaluate your CAC costs based on the months of revenue it takes to recover the cost to acquire the customer, and ultimately this calculation should be net of any costs associated with providing the service on a monthly basis and, even more conservatively allow for a healthy degree of churn. Keep in mind that this calculation doesn't evaluate your true profitability only the capital efficiency of your customer acquisition spend. Hope this helps. Happy to talk this through in more detail and with more specifics to your business.

Check out https://www.balancedpayments.com/ They are made for marketplaces. Airbnb CEO among others invested in them and they have some of the best pricing/payout fees. Also some good info on http://www.collaborativeconsumption.com/2013/10/08/online-marketplaces-are-hard/ One of Balanced Payments co-founders is writing this blog series on marketplaces.

At WP Engine, everything in marketing and sales is included in CAC. Salaries, commissions, coupons, direct advert spend (which you're saying you don't yet have), fees, travel and other costs associated with conferences, etc.. My advice is to err on the side of putting too much in CAC, because that helps you honestly understand the costs. Ignoring some costs just because they don't scale with company size or marginal new customers doesn't make sense to me, it simply means that certain components of your CAC you expect to get more efficient over time. Indeed, they had better! So measure it, instead of ignoring it. You also might find that some of those direct-spend channels are not as inefficient as they seem compared to things like SEO efforts. Or the reverse! All good things to explore of course. I'll also note that at $19/mo in the crowded space of CMS offerings you will find that very few channels will be efficient compared to the revenue you're generating. It sounds like you know that, and are dealing with it with "scalable" efforts like content marketing, however again you should be ruthless in understanding how those costs are really translating into orders and whether that's a financially sensible total strategy.