Ancient Orc you have to farm the Public Dungeons, in particular the Bosses.
For Glass, you need the Fragments, 10 of them, and then you need the conversion matieral, which I think is 10 Gold, and then you get a random chapter from the process. I just bought it in the Crown Store rather than go through all that..
So, either do the above, or pry open the wallet and spend some gold in the Guild Stores.
As for pricing, can't help you there. I personally set a limit and won't buy a motif that is above that price, but that limit can change from week to week.
Prices are tough, for one, Consoles tend to have higher priced items, and not a little higher either, than the PC. Then the Euro servers have higher prices than the NA servers do.
So, first thing you need to tell us is what platform and server are you on.
But, really, all you can do is shop and get an idea of what things list for if you don't have or can use an addon like Master Merchant to track prices. Also, look in Guild Store History if your in any guilds with a store or kiosk.
The ones you mention typically sell in the 5 figure range, 10K to 70K are the prices I see for various ones. Personally, I don't bother with Motifs if I have to pay more than 4K for a chapter I want, or more than 2K for a chapter I need but don't care about. If they are more than that, I don't make the style. Lucky that Ancient Orc and Trinimac are just not appealing to me, as they are the most expensive ones.
I recently purchased the entire Abah's Watch set. It cost me about 40-50k per piece and 100k for the chest. It was aweful, but I've noticed the prices gradually dropping. Some of the more random pieces, like axes, maces, and bows are starting to sell for 30k. I'm not sure how much more the pieces will drop over time, since they are quite rare to get and not many people are actively doing that content.
It reminded me of Akaviri motifs. I paid about 50k for the small pieces and 100k for the big ones. I don't do PvP much, so I bought them off of my friends with massive surpluses of AP. The prices have gone down a bit since then, but they're also quite hard to find on the market. I recommend asking for a friend and looking more around 25k for smalls and 50-75k for the big ones.
This is a PS4 NA perspective.