What is normal for truss spacing depends on how you intend to finish the inside of the building, if at all. The wider the trusses are spaced, the beefier they need to be. The wider the trusses are spaced, the beefier the purlins need to be as well.
If you are not going to put a flat ceiling in the building, 8 foot spacing should be fine. If you are going to put in a metal ceiling, you will want the spacing to be 4 foot or less. For drywall ceilings, the trusses need to be spaced at 2' unless the drywall is going to be attached to strapping that runs perpendicular to the trusses.
When I had my 36' x 48' barn built, I had it priced with various truss spacing. The cost was virtually the same for 4' or 8' spacing. 2' spacing was somewhat more expensive.