The cheapest professional 3D modeling software for the Mac is SketchUp at SletchUp.com. There is a free version and a Pro version for $500. You'll have to determine which version you need but $500 is very cheap for this level of software. $2,500 would be the cheapest full featured CAD software for the Mac.

There are many SketchUp tutorials on YouTube but it will take many hours of dedicated study and practice to master any 3D software worth using.

Here is a link that might be useful: terrain tutorial

Renovator8-
Thank you very much. I am familiar with sketch up and its uses on the free version for designing furniture builds. I was not aware sketch up would be a program available for the landscape portion of what I need. What capabilities does it have for a full acreage (with multiple hills) 3-D view?
While $500 would be a lot just for messing with our house, it is probably rather small percentage wise if it will help best design my barn and other buildings in addition to visualizing the whole layout on our property.

Renovator8
I have been viewing some of the things Sketchup can do with showing a model of my land, and this is exactly what I was looking for!
It appears that it can do everything I will need to visualize changes and move locations of buildings, etc. It looks like it may be my grown up version of Sims :)
If anyone has some other programs they think I should look at, I'd be glad to hear them, otherwise I think I will put Sketchup on my Christmas list!

I also want to sing Sketchup's praises. It appeared a little intimidating at first to me as a novice at 3D design, but it is very intuitive once you learn the basics. I am one of those "knows enough about design to be dangerous" sort of people and so I've been using it extensively as a tool to collaborate with my architect. When I have an idea I'll mock it up and send it over.

I ended up designing all of our cabinets with it, and much of our trim/molding. Our architect ended up just exporting our cabinet models to dwg and dropping them into his CAD file for elevations.

Note that you don't need Sketchup Pro for most of this... it can all be done with the free version (except dwg export/import). Where Sketchup Pro comes in is the addition of Layout, a powerful tool similar to Autocad "paper space" that lets you take orthographic projections of your models and create elevation drawings you can send to contractors. I ended up buying Pro and doing this with my cabinet drawings with my architect's encouragement. If you're just interested in mocking things up and not printing your drawings, I'm not sure you really need Pro.

Sketchup has a huge ecosystem of addons. I've mostly ignored them so far with the exception of "Joint Push/Pull" which succeeds in extruding shapes where the built in one fails.

I am pretty sure you could use this tool to design just about anything.

Cabinet 3d model in sketchup:

2d orthographic projection in Layout for cabinet sub:

While software can let you quickly change ideas, beware of it's huge pitfall. It lets you become emotionally attached to ideas that aren't so great, or that are even poor much more quickly than if your ideas evolved over time with pen and pencil. We see that on this forum all the time. People fall in love with bad ideas and can't get them out of their heads because they've "seen" them with software and don't realize that they violate health, safety, or building codes, or good design principles. Don't fall into that trap. Use software as a tool, but get feedback from professionals before falling in love with your ideas. Remember the old computer saying: GIGO. And guard against it.

Hollysprings makes some good points worth remembering.

There's an old design saying that's also worth remembering, "Your first idea is not usually the best idea!" Good design is a process of exploration and discovery. It certainly isn't a process focused on making the laundry room floor plan bigger!

Just a thought.

"something that we can program our land in and move buildings around on our land. We have a big hill on one side and it goes down to a creek. Something that can show elevations of the land"..... "to help determine pond shape/location, barn redesign and location, Quonset location, orchard, garden, bridges, and some landscaping"

In addition to the program, you need a surveyor. The surveyor can provide all the info you are going to need to model the land. (lot shape, pin locations, contours, tree location (and sizes), creek, etc.
Ask the surveyor what program he uses. He might agree to give you the file. Maybe the program he uses is compatible with Sketchup.

Thank you all for the additional info! I had to laugh today at your response hollysprings. When I find an idea I like (very picky so it takes a while), I have a hard time changing to new ideas. I tell my husband to expect a no and let me think about it for 24 hours...LOL.
Part of the reason I wanted a program like this is so I can be more open to looking at all possibilities!
Naf_naf. Thanks for the suggestion. We actually have quite a bit of land, and there I already have multiple printouts from the county's website, NRD website, showing the land, topographic lines, and flood plains, etc. I have already uses both the county's website and NRD website to highlight various areas to determine total acres in a particular area. I also have a complete soil sample printout for the land. In regards to locations of the buildings, etc ..I was really just hoping to find an all-encompassing program that would show the 3-D effects of any changes in placement/angles/etc, rather than staying within a specific lot line. I was hoping to upload my parcel directly into sketch up, but will visit the county assessors office for a DVD file if I am not able to get it uploaded.