Bob,
I'm currently on a very thin shoestring budget, but have managed to get quite a bit of scenery done. I take it a little at a time. Also, I collect whatever items/ingredients I think might be useful later whenever I can. Materials I collected for trees over a year ago got turned into bushes and hedges fairly recently.
Now if you buy the pre-made stuff...like trees...it will cost a whole bunch. But you can buy lichen and that Woodland Scenics foliage stuff for fairly cheap and use twigs and dried weeds from your yard (or the ditch, or the vacant lot across the way...wherever). I'm always on the lookout for plants that might make good tree armatures!
Also, I was able to cover all of the ground on my layout with one bag of WS Blended Turf (50 cu. in.). That's approximately 17 sq. ft. And that included
redoing several portions. It all depends on how thickly you spread it. If you paint the subsurface a "dirt" color before applying the foam, you don't need as thick of a covering--the dirt patches will look natural showing through the grass. One 50 cubic inch bag of blended turf is about $6 MSRP. You might get about 20 sq. ft. out of one bag...
I would suggest, though, putting down a layer of "dirt" colored foam in the more visible areas. It makes a striking difference over plain "grass."
Get the grass down soon. It'll make a world of difference on the layout! You'll go from that spartan factory look to Scottish moors... Put a few clumps of trees here and there, and things will take another big step forward.
And it's fun to experiment with scenery techniques. You know how I am about trees!
-Rory