Hold it right there citizen...I can tell you exactly how to make good indestructible hills in about...four minutes:
1) Find a suitable height "high density" upholstery foam. This is the stuff you make car seat padding out of, etc. You can find it on eBay in large 6' rolls for around $25. Comes in varying heights, but you'll want to make sure you get some firm stuff (I don't know how soft it gets, but in order to hold big models/terrain...let's not risk it!) You can order sheets in 72"x24" for $25-30 shipped normally. Shop around a little bit. DO NOT go to the hobby store in your town. They sell 12" blocks for like $20...screw that noise.
2) Cut and pluck the foam into the shapes you want. Trick to plucking is sadly...to use your fingers. So it gets pretty annoying, but it lets you get a chiseled/natural edge: Here is the aftermath of pluckery nonsense!
3) Take your hills and spray them with an undercoat of brown (or black around the edges that'll be off-board). Give the top a dusting to cover the colour of the foam, but you'll flock it later: Here's an example of a section of 2" foam that's been cut...plucked and primed.
4) In either order, you want to do two things. A) Apply a slightly watered down glue to the top (I prefer Elmer's Wood Glue max) and then apply flock. You want the glue thick enough to not just disappear into the foam, but thin enough to be flexible when finished. I've screwed this up once, but you'll know the next day if you cocked up. I'd say 2:1 glue to water ratio or so. Brush it down...flock it up. B) Using a cheap hobby paint and a big disposable brush you can dry brush the ridges of the hill that you've plucked.
5) Revel in your cheap sturdy hills. If the flock falls off the next day, you have watered your glue too much and will have to re-do it, but that's fine. These hills are foam so they can bend, be crushed, etc. and they won't break or chip like hard-foam hills will. They're relatively cheap (You get an entire set of hills out of a single $25-30 sheet of foam). You can make 1" tall hills up to maybe...3"?
Here's a set of hills I did this past week, 2" tall...and I've got two large pieces left (so you get quite a bit from a single roll).
PS: To make them look better you could mix in a variety of flock to colour it more naturally...I was lazy and only had one type of flock on hand.
PSS: Here is a picture of the 1" hills in game. The new 2" hills will be used to elevate and build more variety. In most games, the 1" hills pictured denote our wooded areas (area cover)