Low volume production will increase cost, and amortizing the cost of development over a small number of units produced can inflate prices massively.
However, even recognising that, the cost of this DAC seems excessive. Benchmark are also a relatively low volume producer I am guessing, I also suspect that their production also relies on non-automated processes. Yet they cost an awful lot less and clearly they are the result of development and design of the highest order. I have no idea how much it costs to make one of these TotalDAC products but the only way it could get anywhere near justifying a 13000 euros price would be if product development and production is grotesquely inefficient. I suspect it is more likely that the manufacturer knows there is a niche market which equates price with better sound and exclusivity and has just positioned themselves to serve that market. From a strictly business perspective it is quite a smart move.
At a deeper level though, if you have a product which has been commoditised in the way that DACs have been (as evidenced by the Apple dongle or the multitude of excellent DACs costing less than a twentieth of the price of this) then I think it hard to justify the price of this unless it is viewed simply as a statement and audio jewellery. The problem is that it is built like alternatives costing a few hundred dollars or less (actually the March Audio DAC has much better design and casework for a tiny fraction of the cost and is also made by a small supplier).
In this segment price is meaningless as nobody buying this is applying rational analysis of performance and cost, but given you can buy some lovely audio jewellery at that price I don't know why anyone would buy this.
Don't disagree with anything you said, but when the company principal and maybe one or two other people do all the work in a high priced Western economy, they have to make a lot of profit on each unit to make a living. It's not just "cost". I'd assume payback per hour for Vincent to decide it is worth it to employ himself is very high. I don't think he's doing this to live at minimum wage. And as you said, in this market sector you can pump the price up and it may actually increase your sales. For certain segments, a higher price increases sales, instead of reducing them. His least expensive DAC is not his best selling one.