Price scaling is pretty basic, it just uses a global table which it applies to all items if enabled. (except Space credit but that's another topic) You can edit the table, but it will effect the rules on scaling all item prices. With ores you will eventually reach an "upper limit" on price particularly if most people are selling not buying, at which point the price remains pretty flat unless someone cleans out a large percentage of the stock. At that point you could increase the base price of that item and it would flow through to the scaled price - but then potentially you are throwing the low stock price out of wack which players may exploit. Not to mention an ingot requires a certain amount of ore, which was used to set the base price of its ingot. If there is too much ore the lower price may make it cheaper to refine it yourself, which avoids "dead markets"

I originally wanted to write back the price changes using a different algorithm, but that meant you had no baseline buy/sell price benchmark, which meant items either became immediately worthless or super valuable - with no way of rebalancing the price, which meant it was very easy to exploit the market and take all of its money with rogue trades. - Just buy all the stock and wait a few minutes, sell it back, wait for the price to drop, rinse/repeat.

The other system which was going to be a parallel system was where trade zones overlap players automatically buy the cheapest market, and sell to the most expensive, as a sort of stockmarket thing. We also considered an actual stock market - but it was sidelined in favour of other features.

Pretty much your options are:
1: Disable price scaling
2: artificially inflate the price and stock of an item you wish to remain static priced, give it so much stock they could never burn up all the stock and manipulate the price.
3: use a player owned trade zone instead for the items you dont want to scale which has no price scaling at all - and can have stock limits applied.

You could choose not to trade an item in the npc market using blacklist, and then create a player trade zone that overlaps which does stock the item you dont want price scaling, but I think we setup blacklist as a way to prevent trading at all of an item if the NPC doesnt trade it.

Realistically then your only other option is to have an NPC market with no stock, that pays/costs 0, and an adjacent player owned trade zone that does have stock and prices. The player zone's price will never change no matter how much stock, and you can limit how much to trade too. Nobody will trade with the NPC because he wont pay anything so they are forced to use the player trade zone.