Determining the value of a card by itself is step one of arena. Not that hard, just requires playing a little bit and the help of some simple spreadsheets, like the one you linked above. The next step is determining a card's value within the picks you already have. This is a lot harder than it sounds. How does it fit in the curve, deck archetype, and what synergies does it have? Do you have enough of that type of card already? Or do you not have enough for 30 cards, but since you're at pick 15, you're likely to get another down the line?
Heartharena.com might be able to help you with this. You might disagree with some of their ratings and how they weight certain curves and synergies, but it will at least help you realize some of the nuances that go into making pics beyond "which of these 3 is best?". Also, the guys that made heartharena have a stream, arenacoop, which they draft and play arena agonizingly slow. But they explain almost every decision they make, so it's great for learning.
For antique healbot, I'll say that it's rated low because if you need that 8 health to live, that healbot is likely only a delaying tactic. Your opponent likely has board control, will easily kill off your 3/3, and will remove the 8 health in the next 2 turns. Plus it only leaves you with 5 mana for that turn, which isn't much to work with when the opponent has 10. That said, it does seem to be quite popular and it's fucking annoying when someone plays one. But it's just that, annoying. Played properly, healbot in arena just delays. Constructed is a whole other matter.
