As you probably know, the factory-produced diatonics go as low as low-low-F. Besides, they are only made in Richter Major tuning. With such a low key, richter tuning really stops making any sense. You get 2 chords + some soloing possibilities in the mid octave. The draw bends become unusable / unmusical.
With a spiral tuning, however, I could get 2.5 full octaves in harmonic minor - using blow/draw notes alone. For low-low-D, that makes a huge difference!
This low-low-D is labeled by 1 draw (the root note). So by regular standards, it should be considered a low-low-C (if we label by 1 blow, I won't use the term "straight harp" since positions no longer apply with that tuning).
I've managed to go so low by cutting two seydel1847s in parts and making a 12-holer from them. Not the most sophisticated method, but it worked. It's not very good for airtightness in the low holes, but I was making it for myself, and I can play that - so who cares, really...
Several months after recording the Toccatta, I've made another low-low-D. With significant impovements this time. I've used another tuning (circular) to get a natural minor / major scale on open notes. I have also made it using a single solid (not cut in several pieces) seydel 1847 reedplate. 1847 is a great, durable model, with only one huge problem - it comes in LowC as the lowest key. And I was building a low-low-C. Retuning down by -12 semitones is not really an option... So I've filed all slots longer to fit lower, longer reeds (left to right the reeds are: 1,1,1,2,2,3,3,4,5,6). The blow reedplate is fairly easy to mod this way, but for the draw I had to change the position of the rivet (screw) hole. Each such operation doubles the chance of destroying the reedplate and starting anew.
So... speaking about a possible order for such a diatonic. I definitely wouldn't make it like the first one I've built. It is a wild mustang that only I can ride, and you will hate me for selling such a diatonic.
If I were to build it like the second one - the materials needed are three LC 1847s plus a SeydelFavorite LLF (or thunderbird) for 1 blow / 1 draw). This all will be destroyed to create the LLD, so you count how much the raw materials will cost (even if you have a dealer discount, that's quite a lot.) Then add a week's worth of work, and some high possibility of spoiling it. I'd say it would be about 600EUR.
I took some pictures while I was building it.