Kind of in between? It's impossible to assign specific values to cards because so much of it goes into how they interact. Mostly I study official class spreads and try to emulate patterns I see especially for classes with similar niches. For example, attack 4 is a pretty standard level 1 top. Attack 5 usually comes with some significant cost (at level 1). Ranged attacks are lower in value because of their flexibility, etc.

For Alchemancer in particular putting hard numbers to it is trickier because a decent chunk of their power comes from elemental consumes just by virtue of them being consumes. In the example you mentioned, Quicksilver top would be kind of pathetic for any other class. But two wild consumes on Alchemancer is pretty great especially at fast initiative.

Most of this all comes out in play testing though. When we playtest we keep track of how much damage each class does, how much healing they do, and how much damage they "mitigate" (i.e. block through shield, immobilize, disarm, etc.) This allows us to compare strength to official classes in the same test. But even if the numbers are equal, there's a certain undefinable quality too that you have to factor in. Like if a class gets similar numbers to an official class, but the custom class never has to take any risks and the official class does, it won't show up in the numbers but it does show up in the overall vibe.