I either lower the exhibit or raise the viewing area. You get same effect, but the cost will be cheaper depending on your terrain elevations. On a flat surface, for my particular method, it's cheaper to raise the viewing area.
I put a 3-square wide "berm" for the visitors with a path in the middle square. I use the low post & rail fencing cause it's a whole lot cheaper than any of the Dino fences! Also, there's no problem for the visitors in seeing the animals. The low stick fence seems to block the view. If you get problems with Dinos feeling crowded, switch some of the fencing to a tall solid fence. Some of the really large exhibits may also get a raised viewing area.
How deep? All Dino's get lowered by 2 levels with these exceptions:
T. Rex 3 levels
These can all go with one level down: Loch Ness Monster (Bonus critter), Ankylosaurus, Coelodontia, Deinosuchus, Kentrosaurus, Meiolania, Plesiosaurus, Stegosaurus, Styracosaurus & Triceratops
Remember, you don't need to have a ramp for the keepers or scientists - they can handle any drop or rise in elevation.
Between two adjacent exhibits I raise the ground by the needed level, with a one-square wide berm. I put the cheapest fencing on both sides so there's no complaining about steep elevations, etc. I also usually have one scientist for every 2 Dino exhibits, (some of these guys sure make a lot of dino-guano...) and you can put the exhibit doors here across from each other, and away from crowds, buildings, etc.
I use the fences forever hack, but even without it if your fence is damaged the Dinos can't escape.
Alex