It's fairly linear for the same kind of truss, like say a 15 foot span to a 20 foot span - just think bigger triangle. Past a certain point, in order to support the shear/wind/snow loads across a span requires increasingly higher grades and width of lumber and you start getting into non-linear territory. Pole barns are an extremely common order for truss plants, and that size should be pretty standard. In single family houses, though, you start getting into "features" pretty quick which affect the price quite a bit, think ceiling trays, HVAC platforms and so on, so you're pretty quickly into some nonlinear territory there as well.