I think you are 100% spot on with your initial calculations, up to the point of the 6 figure domain sale"s", we know there has been at least one.
If there has been
over $1M in
revenue for sellers in the last 18 months, then a generous calculation of the addition the 30% commission plus logo fee would equal approximately $1.5M in overall
sales. That equals $1M in sales a year. The same as in 2013...
http://tldinvestors.com/2013/06/brandbucket-says-2013-on-track-for-1million-in-sales.html.
Deduct 1 six figure sale ($1M - $100K = $900K) and assume that the remainder sold at a $2K average, that's 450 domains sold a year = 37.5 a month.
So my calculations are pretty close to your calculations Svede and I think about right for the amount of sales we see listed in this thread.