@Jon-Mitchell tangible what you mean. And yes if you have a limited product which is high in demand the prices go up. Problem is with the way our economic works. Those prices are established over many years/instances. So I don't see much leeway or intentions for them to change this.
Besides on a sidenote. Translators from literary works earn less than for example ones that specialize in official documents, etc. Although I can only talk from the standpoint in Germany as I know some translators personally. Translators in the literature business earn around 13-15k per year. Although it depends on how much work they do as they usually earn per paragraph (usually a paragraph is 55 keystrokes).
Sure, the profits will go up even if the prices are falling. but it will not be so much if we take into account the prices of the licensing, marketing, translation, typesetting, etc. A big share goes to the original license holder. But as I can not recall how Sam licenses them: if it is a share of units sold, which is highly unlikely as this would go against good business economy. I assume it is more JNC needs to pay a agreed on amount for the right to bring it to another market and then the japanese license holder gets surely a cut of the sold units.
And what follows is my speculation so don't take my word for it. It is just an example why I doubt the loan for a TL will go up significantly.
Let's assume you make an offer for a LN to the original publisher. If I recall correctly Sam licenses such things per Book, not for the whole series. So we have Book A Volume one which costs initially 10k (and I believe this price could be even higher). Now we take into account that it needs to be translated. a norm paragraph is around 55 keys so any TL earns around 1.80-2.50 Bucks per paragraph (and nope it won't help to separate the speech from the normal text usually). Now we take into account typesetting, marketing, distribution, cut shares for other plattforms liek Amazon, B&N, etc).
Let's assume the example of $10 per book. After all this I assume JNC earns around $3 per books which goes slightly up the more they sell this book (seeing as the initial investments stays the same, atleast in the cases of ebooks).
Now book prices go down. So if it is 2 bucks cheaper you would maybe only earn $1 per book. Sure you would have a wider market with more licenses but not really much more money out of it. And just in case anyone things that in this example they should leave just the original prices. Sure the publisher would like to do so. But as the market gets saturated with more and more licenses (and publishers) they most go down in prices.
TL;DR: There will be not much fluctuations in the amount a TL can earn even if there are more licenses. The market just doesn't offer this leeway.
And as mentioned this is my take on it with not verified prices. And I highly recommend that we should continue this really in another thread as the initial purpose of this one here was if a translator should be allowed to translate more than one series. Even though with all my rambling I should have made it clear that in my opinion he/she has every right to do so as those equals his earnings.