"Well now he knows 100A would be very low (rip-off) and that it can go a lot higher than that." - Does he? Is it? If such an item goes for sale and only receives a max bid / single bid of 100a (perhaps from you) should he not sell because you believe that's a ripoff? If there is no buyer above 100a please tell me why its value is more than 100a. Buyers and sellers set prices with there subjective value of the item compared with all other items. That price discovery doesn't take place until the sale, hence you always end up back at "sell and find out".
"not completely out of the blue either" - So just mostly out of the blue? really useful stuff. Past prices are no guarantee of future ones, especially when the comparison is between core skins and non core skins and even more so in market with so few participants. Pleikki said Item A is selling for X, Item B might sell for Y.
At any given time Is there:
1. The same / less or more buyers for strength Amber as "core"?
2. Do those buyers have the funds to make a purchase in that suggested range?
3. Are those buyers motivated enough to participate in a bid war and push the price to the suggested range?
You don't know the answer to those questions (nobody does) and yet that is exactly the kind of data you would require to make a prediction of the result. and yet you dishonestly make the prediction anyway.
"could be an indicator this would go quite a bit higher" - The reason you phrased this so vague is because you know its 1 factor in a multivariate equation to which you cant quantify almost any variable. Hence you cant accurately predict the price (so don't try).
"Though we cannot know whether a bid war pushing an item very high occurs or not" - Thanks for concluding with my argument 
Saying core skins can sell for 100a is valid as some vague ballpark number to prevent madlad lowballs of 50e being accepted, fine stop there. Trying to use that to infer that "250a+ is possible" is not appropriate or helpful, anything is possible.
Prices don't always scale with rarity.
Prices don't remain consistent over time even for the same item.
Prices of similar items aren't always similar.
Prices of very rare items can never be known with any accuracy.