Standard base and case molding are not much different in cost that good quality 1 by 4 pine. Using 1 by 4 will give a more rustic look. If you are going for cheap, use MDF (Medium density fiberboard) base and casing. Most lumber yards carry it, also the big box stores.
Unless you want the look of flat stock, it will be less expensive to buy a paint grade , finger jointed profiled molding from the lumber yard. Unless you are looking for a log cabin rustic look, you will have to buy #1 pine or poplar stock; lesser pine grade will be full of knots and is not suitable for painting.