The question is about interior windows trimming.
What kind of wood is the best and economic choice?
I need to cover 2x4 studs and then trim around. This is new contraction windows.
Thanks,

The cheapest is probably 2 1/2" colonial casing sold at Home Depot and Lowe's. It's clear pine and can be stained or painted. You miter the top corners.
You can also buy solid white vinyl in the same style but I think it costs more.

This is what I have to do.

That window isn't installed yet? It's in backwards?
Lumberyards sell the casing trim material too, either clear pine or vinyl, in the width you need. Just cut, put some caulk on the back, a few fine nails, and put the trim on, with a slight edge of the casing showing. If you want to get fancy with a sill (stool cap) you can, or not.

What is on my mind, what would be the best and economical to cover the studs. This is what I have to do first.
I'm planning to build the whole "frame" and place it in to the opening. I was thinking about vinyl.
Windows are installed already.

Window is in right. Install a sill, trim sides and head with stock or ripped stock and then colonial or clamshell casing depending on the rest of the house. Cheapest solution would be a cut drywall return and edge with no trim. That gives you a contemporary finish but is not my style.

Brian has set for you the nicest appearance.

Install a window stool and apron. Box window top and 2 sides with 1 by 3.
Make flush with S/R. Trim with casing. Paint or stain.

Chuck