While I'm not on Midgardsormr anymore, the issue of the weapon prices can be easily answered.

To get a -reliable- way to obtain demimaterias of any kind (whether they are primal/battle/field demis), you need to desynthesize your way to 90-100 in the corresponding classes, and THEN desynthesize what will yield the demimaterias. When primal weapons are concerned, we're talking about ten battlecraft demimaterias and three primal demimaterias.

In order to profit the most off of your synth, you would rather obtain those demimaterias by yourself. Before I left there were plenty of relatively cheap Ifrit demimaterias on the markets, but the supplies for the other ones were very low.

In order to obtain the demimaterias you'd have to farm the primals in question and then actually win a weapon so that maybe your desynthesis will be successful and have a -chance- of yielding 1-3 demimaterias. The only reliable way to get them is to have a full party of friends and have every desynth involved with the weapons spread out amongst the members and then pool all the spoils together.

This takes time, and farming Moogle HM is as dull as it gets. So as a crafter when you finally have all of the things and it's all said and done, you try to make a weapon sale and people won't understand why you're not selling all these ingredients synthesized into a shiny weapon at marketboards value + 100k gil, as opposed to the price you had in mind for all that time and effort you put into getting the damn things and/or leveling your desynthesis skills. So you cut it a bit, and then a bit more, and then a bit more, until finally your weapon sells for less than you had anticipated.

Now pair up that phenomenon with the fact that the Midgardsormr population either wants to hold onto their gil or doesn't have any in the first place, and that's why the economy for items that have a bit more rarity to them is terrible.