Perhaps the best way to think about and focus on marketing costs for your two-sided marketplace is on the supply side. i.e. the wine makers you have to recruit to your platform.
This would be especially appropriate in the first year. If you think about it, once passionate and innovative winemakers are geared up and ready to offer their product - a compelling story with incredible wine rewards attached - the buyers will line up. The reason? Because those makers will generate their own marketing and press to bring in backers. In the Kickstarter model, the project creators are the engine that keep growing the platform. And their press will bring press to your wine platform.
As far as the COCA for bringing in wine makers, there are a number of different channels you can try to quantify. If I were in your shoes, the only online channel I would consider would be targeted Facebook ads. You can build super targeted audiences of people who are wine makers, restauranteurs, owners of vineyards, etc to put ads in front of. Just make sure you have a beautiful optimized landing page for them to land that prompts them to leave their info. Then you follow up and sell the concept.
COCAs through FB ads can wildly vary, but it's quite possible you could get each qualified lead for ~$100 once you've optimized your ad creative and the audiences you target. I just finished working with a company that does just this who I'm happy to connect you with if you want a better point of reference. It's tough to shoot numbers from the hip, but these guys do this for multiple startups and venture-backed companies across a bunch of industries. They could probably give you a good sense after a 15 min conversation.
Outside of online channels, maybe you've also thought of repeatable face-to-face ideas that revolve around national & international wine festivals. (maybe for your first 10 wineries you personally tour a wine-rich region and meet the makers)
Finally, on top of all these factors specific to your wine platform, you can factor in the more standardized costs associated with email marketing and biz dev. This is especially appropriate when the platform is growing after yr 1 and you have a sizable backer list.
Hope that's helpful. You're welcome to message me for clarity or to expand on anything. Cheers! p.s. I love the name Cruzu