What is the most cost effective Warhammer box set based on size and number of models?
40K-wise, there are a few builds that you can do that are relatively cost effective. 

Any of the marine variants tend to get you a full army for the least cash.  Xenos armies tend to have much cheaper point cost units that are still equivalent to Marine cost in cash money.   Of the marines, Space Wolves and Grey Knights are probably the cheapest to build.  They have some high points cost units that are worth every point in competitive game terms.

The Draigowing Grey Knight build is the most cash efficient list out of those two options.  Draigo, a Librarian, and two 10 man squads of Paladins will cost you about $250 and can be up to 1850 points depending on how you kit the Paladins out.

I'm less familiar with Fantasy, but I've always heard that High elves tend to be a good balance of cash cost and points cost.  Ogre Kingdoms might also be a good choice given their lower model count.  However, even a cheap Fantasy army will cost you 3x as much as a cheap 40K army given the higher model counts required for that game.

EDIT:

Since on re-reading the original post, the question is about models and not armies I would say that most of the plastic box sets are actually fairly equal in value.  You usually get 10 small models or 5 more complex models for 25-35 dollars.  Older kits tend to have less of the trimmings; newer ones(such as ones for Blood Angels, Grey Knights, Dark Eldar, and Necrons) have more alternate upgrade options and tend to let you build more than one unit type from the box. Plastic Kits in the 50-60 dollar range all tend to be larger more complex models.  

The only place where you are probably going to get a little more for you money is with basic transport vehicles.  These are usually costed a little lower at the $35 price point.  Though these tend to be the most boring of the vehicle types.  Basic Fantasy troops boxes will also tend to have more models in them at the cost of simpler sculpts.  Battleforce boxes can also save you about 15% over buying models individually.

Anything Finecast is a little pricey and you wont get as much model for your money.  Unfortunately some of their best looking models have been converted to Fincast already.

GW is pretty consistent with their pricing in this way.  If you see two boxes both marked at $35 you're probably going to get a pretty equivalent value--slightly skewed towards the newer kits.  GW's basic product value isn't so bad, it's always been the amount of things you need to buy to play the game that's the trouble.

So, unfortunately, the answer is whatever you think looks coolest ;)