It's a good question. I don't know much about it, but if we could get answers to some questions, it would be possible to start evaluating the price of this awesome gear.
If I were to make you Steelsilk, here's how I would factor it:
For sake of example, let's look at the
Imbued Steelsilk Breastplate (Quellious)
1) How many hours did it take to get enough Terrorantula Silk for a particular piece? 1k/hr is a pretty standard rate of plat gain from camps, and as this is a pretty nuanced camp, I would charge that much for the silk swatches.
For three swatches, that's 6 silk. I would estimate based on numbers pulled from my ass that it might average 3 hours of time spent waiting on the right conditions to spawn Terrorantula and getting him to drop enough silk. (if anyone has more accurate info on average rate of silk per hour, please let me know)
Price for Factor 1 = 3k
2) Getting together all the other little pieces. You need an enchanter to summon you the vial of mana. you need a cleric to imbue the topaz. Buying the items off a vendor. These are all pretty cheap, but again the maker should be valuating their time at around 1k/hr.
For the sake of ease, let's say it takes an hour for them to have collected all these items and hit the combine. But they could also have multiples prepared, cutting down the time per item, so let's say that these other materials run 500pp in cost of goods + prep time.
3) How much does a tailor value their skill on p99? 167 is pretty expensive to get to if I am correct. So they definitely deserve a premium for this skill (again, it's a vast amount of time they have sunk into the boring farming of mats and risking carpal tunnel).
If it were me, I would sell the bp for no less than 4.5k. I'd seek more like 5.5 to 6 given the quality of stats and the fact it can be slapped onto a level 1. Compounded by the fact that you're seeing a lot of Demand with very little Supply, it's possible to hike the price up even further.
Plat is easy to get. Terrorantula silk is not. Plat can be fun to farm. Tailoring skill is not.
Of course, those numbers are based off of total guesswork about how hard it is to get enough swatches, but just looking at the stats I would say it's totally worth it to drop that sort of plat.