A great many things go into the True Value of a card. Here is a few schools of thought and others can add on as they deem fit:
The Hey I know this person!!
Common (C) .1-.10
Uncommon (UC) .5-.50
Rare (R) .10-1
Ultra rare (UR) .20 - 1.50
Epic (E) .50 - 5
Legendary (L) what ever the MV is
Hey this person is big in the community for XYZ
1-2$
Hey I have no clue who this is they've done nothing but lived under a rock and somehow managed to become an epic named
9003 but there market value (MV) is high so it must be good!
It probably isn't and these are either a blessing or a curse. MV is just a number as the computer tries to figure out how popular a card is. As such it can be fudge rather easily. but is also super noticeable. When the sales chart looks kinda like swiss cheese with sales all over the place the card might have vaule but only to one person. If there still pouring countless bank into it you hit a gold mine! if not you got a good MV card
It's got an Ex- flag
CLICK IT if the nation is alive behind the card its worth about .02-.03 cents more than junk value
If its not alive behind it and the screen gives you a lovely red message that says hey there dead. That card is worth about .50-500+ (yes 500+ see the card leader board) based entirely on what rarity is is and how many owners it has.
1-2 owners on a common is worth .50-3
3-4 owners is .25- 1
5-8 owners is .30ish
And of course the massive disclaimer that everyone will tell you the card is worth what ever someone else is buying it at. (Not helpful I know, but none the less true)