The thing is that Sorcerer can produce only a single unit at a time, always, on the summoning channel, and the problem here is what this unit brings to the table. Horrors bring a lot more for x2 time the mana then node serpents do, so they are more worth while the investment, simple.
Now you are talking about interesting things, but you don’t go far enough : what would happen if 2 node serpents are worth more than a horror ?
The thing is, that is true for ALL sorcerer units but the apprentices, because they are summoned, and you summon only one at a time. So either you make the lower tier too good for their price and you don’t need to research further, or you leave as it is, each tier being better than the other, albeit more expensive, and at each tier you mostly forget about the preceding units to focus on the new one.
This problem has no solution in the current sorcerer paradigm. No cost change and no buff or nerf will ever change the problem, because the problem is that you will always use the best unit you can, and there will always be only one best unit, unless you hyperspecialize all of them, but hyperspecialization in AoW is considered worthless, and the node serpent is a perfect exemple of this.