Sometimes you get lucky and nature fills a void for you. The neighbor of one of our members was trimming a bush and the cuttings looked like little trees. A quick spray with adhesive and some ground foam and "instant trees." Cost: Near zero as they used little foam from the bag.
Another "bang for your buck" is using "silver" construction sand for ballast. You get 100 pounds for about ten bucks. It's easily paintable by shaking it around in a coffee can while spraying your choice of primer on it. It's a little larger than true O scale ballast, but smaller than chicken starter grit or aquarium gravel.
By the way, the road is asphalt roofing paper. A huge roll is about $35 but that will give you enough for all of the roads for your layout, plus all of the roads on the layouts of several friends, your club, the local train museum, the layout of the H.O. guy next door you didn't know about (you get the picture.) We still have about 2/3 of the roll we got after we did a pass on some of the roads that needed it, plus some roof tops and a parking lot. We're planning to change some roads and aren't planning to get another roll -- we won't need it. It cuts easily with ordinary scissors.