Please keep in mind that because prices are constantly fluctuating, it is hard to pinpoint the absolute cheapest way to level desynth at any given moment. To that end, in most cases, I try to point out multiple items at each level, one of which should be the cheapest. Please compare their costs on your server to determine what’s actually cheapest!
Remember that you will continue to gain desynth experience on every success up until your skill is 25 levels higher than the item. Then, you have a chance to gain experience up until 30 levels higher than the item, at which point you stop gaining experience. When it’s best to stop using one item and swap to a new item is hard to determine, so I’ve often left that choice up to you by overlapping the suggestions. Generally speaking, the more sacrifices you leave for RNGesus, the sooner you should swap. The more you fear the wrath of RNGesus, the longer you should continue with a given item before swapping to the next.
For LTW Raptorskin ring is what i went with and am close to 70 now. It seems to be the cheapest and easiest if you grind mats for it. Wildlings cesti (monk weapon) and the militia choker are a couple of one star i55 craftables that are cheap compared to others. After that if your strapped for cash then any end game dungeons are better to grab items from than crafting but that goes for most gear desynth classes I believe.
For CRP, leveling was easy as I swiped all the housing goods before patch so prices didn’t skyrocket yet. Bloody Lance heads were cheap as well so grab those up and go at it. For profit, look for cheap “manor” items. I desynthed the stools which were going for 10k a piece and sold the wax which was going for 85k a piece. So far 1/2 chance you’ll get it so that’s a plus.
Militia spinning wheel was next for me as I was able to gather spruce lumber pretty easily and cost wasn’t too high. past that you just have to hope for lancer weapons and bard weapons to drop in dungeons.
I will confirm that 3 star housing items do generate Field 3. However the rate is terrible, even with super cheap base materials it will still put a much higher cost vs than using relish. It could be argued though due to how jacked the eco is and a lot of fast turn high gil combos are out there now for the 3 star crowd that this option is still worth doing just to save time by throwing economic leverage at it. You are looking at say around 1 field 3 per 15 3 star gsm housing items nuked.
I only started desynth today and got my BSM up to 67.40. Not really sure if this is particularly good as I have nothing to go by in terms of speed. While I was at a loss, I started off with buying weapons from the vendors as they have been cheaper then the market boards. This was mainly to understand it.
I think your best bet until ~75ish skill are Wolf Winglets, craftable in the PvP section. They seem to use the cheapest materials to me. Remember you can purchase Aldgoat Horns from the Kobold Vendor for 238 gil each. These can also get you Battlecraft Demimateria IIs, which are useful.
Militia Sledgehammer is just as cheap, I think, and gets you Fieldcraft IIs if you’re looking for those. Since I have plenty of them and want more Battlecraft IIs, I’m planning to focus on the Wolf Winglets.
After those two stop being good for you, you might want to get into PvP. You can get i70 Warwolf Blades for 965 Wolf Marks, which isn’t too bad if you enjoy Frontlines. You can also spam Ifrit as a PLD, WAR, MNK, or DRG hoping for the weapons for those classes. There seem to be zero good crafted options for BSM at the i70 level, sadly. Your only crafted options are the i70 weapon parts, which can yield Fieldcraft Demimateria IIIs, but require 9x Coke each.
Soon, once prices stabilize a bit more, I’ll plug the seemingly cheapest desynth items for each type of demimateria into a spreadsheet of mine and get the actual costs on my server. Doesn’t help other servers, but hey, at least it’s some method of comparing the items! We need what, 80x Fieldcraft Demimateria Is for all the new DoL/DoH accessories, plus 110x Fieldcraft Demimateria IIIs for the new tools? So, yeah, I can definitely see finding the cheapest ways to get them to be useful to know.