A card does not have a price. A card in a condition has a price, and most of the disappointment in this hobby comes from confusing the two.
What moves the number
Which printing. First edition, unlimited, a regional release, a promo. These look alike and are not alike.
Set and rarity within that set.
Centring, which decides more than most beginners expect — and corners, edges and surface alongside it.
Raw or graded. The same card in a slab and loose are two markets with two sets of buyers.
Why two of the same differ
Because one was shown tilted under side light with the back visible, and the other was shown flat and front-only. The visible card wins, every time. How cards are graded explains what graders actually look at.
How to find out rather than guess
Compare completed sales, matched on printing, set and grade — not on the name of the card. A graded 9 and a raw copy of the same card are not comparables for each other.
For raw cards, be honest that you are comparing against an opinion about condition rather than a fact.
On predicted grades
Never price a raw card at the value of a high grade you have not got. That number belongs to a different object, and a buyer who paid it will say so.
The live answer
Cards price fast in a room because the people in it know the set. Start low on anything you are unsure of; the bidding is the valuation. How auctions find a price.
Questions people ask
Is it worth getting a card graded?
Only when the likely gain clearly beats the fee and the wait. For most cards it does not.
Does a sleeve protect value?
It protects the card, which is the same thing over time. Storage is part of worth in this hobby.
Are bulk commons worth anything?
As lots, yes. Individually, rarely worth the camera time.
More: pricing trading cards, packing cards.