That's kinda what I was trying to say earlier.
The only way to really assign value is to sell them, and see what they bring.
The general guidelines on a lobby card are as follows:
1. Popular titles command better prices
2. Title card is generally worth significantly more than a scene card.
3. Cards with portraits of the actors are worth more. Value depends on popularity of actor and also popularity of movie.
4. Cards with iconic scenes from the movie are worth more.
5. Scarcity.
6. As always, condition.
As an example, I posted a lobby from North by Northwest where Cary Grant was being "shot" by Eva Marie Saint in the Mt Rushmore restaurant. It is a notable moment in the film, and worth around $100. However, there are two better cards I don't own. The one with them running on top of the Rushmore statues will go for $200 or so, and the one with CG running from the biplane will go for $300-500. There are 5 lesser cards that go for $50 or less. The worst are known as dead cards, and these don't have any of the stars, and usually portray wide angle scenes. Generally bird cage liners.
Of course, it is all a game of patience, and they will all sell for exactly what 2 people are willing to pay on any particular day. Bottom line, is buy cards because you like them and can afford them. It is kind of given that you will break even overall (at best) when it is time to sell.