Are you comparing old pewter models to the new plastic ones? That is not a fair comparison. Besides that the plastic models are more detailed and tend to be bigger, plastic models cost a lot of money to get started. They are cheaper in the long run then pewter, but the cost of creating and mandating plastic molds is really, really high.
GW is not going to make back the cost of character plastics by selling a lot of them. (People typically only need one apothecary in their army for example), so they need to be more expensive to offset that cost. A tactical squad isn't $35 a model, as they will sell a lot of those kits, so the price can be comparatively lower, same reason razorbacks and Rhinos are around $40-50.
Pewter models have really cheap molds to create and maintain, which is why the older pewter models can be cheaper than newer ones. But, it's still a supply/demand situation too.