Joe Davies lays it out in this video 
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Viability…
There are 5000 CrossFit gyms [in the USA]… if you can convert 1% of those 5000 gyms… that’ll get you 50 clients.
To start, offer a low-commitment entry service like building a high-converting landing page and running ads.
Charge $500–$1000 for the trial run (including ad spend) and guarantee a certain number of leads to build trust 
Get those results, then upsell to $1000/month retainers.
If all goes well, you can add on other services (eg. SEO, GMB optimization, etc.) and charge more 
Build case studies of your most successful clients, leverage those to land others.
If you’re struggling to get any takers, Joe suggests doing the first 2 free of charge.
And if the CrossFit niche doesn’t tickle your fancy, you could always try this approach with another niche 