Looking to put one of these on a 6.8 AR for deer.  That's all this set up will be used for, good choice or is there something better?   Want to keep it under the 500 dollar range.  

I just bought a Leupold vx2 4-12x40 cds on eBay , new shipped for $270.

You get a free custom dial from Leupold with it.                        

I bought it for a Bushmaster .450

The magnification is dependent upon your hunting terrain.  Where I hunt I'm in thick woods where 3 power is the highest I've used. 

I keep a Leupold 2x7 on my deer rifle, 3x9 on my elk rifle. The 3x9 VX2 leupold should serve you well.

I have a very similar scope on my 6.8 deer rifle and like it so much that the brown truck of happiness just dropped off another one today. It is the Leopold 110686 VX-R 3-9x with Firedot. The red dot on a lower setting is nice for the first and last half hour of hunting, especially on a rainy day.

The best deal I found was here for just under $400 shipped.

It is a great scope for t6.8 at 200 yards or less. If much longer shots are anticipated, you might consider the 111236 with ballistic Fire Dot. I thought abut going that route, but 200 yard shots are pretty rare in the woods around here.

That's a great price for a CDS! I've been wanting one for my .308.

A VX-2 is a great scope. I suggest looking at the 2-7x if you can for comparison. We have 3 on our guns for Texas white tail.  
The 3-9x is my second choice but still a great deer scope.

Checking that one out thanks. 

To my way of thinking, thats a bit too much magnification.  The field of view on most 3-9x scopes runs about 30-35 feet at 3x.  When deer are in close, and ranges are short in the woods, that s not a lot of FOV.

On the other end at longer ranges the 9x is fine.  However, I've never felt handicapped shooting anything at ranges to 200 yards with a 6x.  I don't regard the 6.8 as anything close to a long range cartridge.  So I'm not getting the need for anything in the 9x range.

Personnally, I've used a 1.5-5x, 1.75-6 or 2-7x on all my whitetail rifles.  I really do think you'd be far better served with something lower magnification.  The VX2 2-7x has a low end FOV of 44 feet.  Thats substantially larger, and makes for far faster target acquisition.

I know you said "sub $500".  Look into Leupold VX3i in 1.75-6x. Its $100 more, but the VX3 is a big step up over a VX2.  You won't notice the difference.... until its the last five minutes of legal shooting time and you are trying to acquire that big buck in the shadows....

I have a VX3 3.5-10 on my 30-06. The buck I killed with it this last season was 35-40 yds out. Had it set at 3.5X as that spot doesn't offer long shots. I had no problem with too much magnification; your 3-9 should be fine

In my experience and type of hunting land around here a 3-9X may be a little too much magnification.
I prefer some with 2X or better yet 1.5X on the low end and maximum of about 6X at the high end.  
I think I shoot 90% of game with the scope set at 2X

Don't overthink this if you have a open terrain and cover that you will hunt then have both, have a variable 2 to 7, 3 1/2 to 10, 1 to 5 it doesn't matter as long as you think you've got yourself covered.    Two things I will mention. The first is if you have too much power your eye relief gets funky. And secondly sometimes you may find you have simply too little power say if you were sitting on a power transmission line.   This gets apparent when you get used to playing with target rifles with 6-20x scopes and then go hunting with an1.5-5x.  That said you should to make any reasonabl shot within your skills with a fixed four power.

Hmmmm. This thread got me thinking. The 2-7x VX-R with Fire Dot # 110684 is 2.6 oz lighter as well as the larger field of view at low power. Since my 3-9x never gets turned up much past 5 or 6x, I may move that scope to another gun and buy a 2-7 for my 16" 6.8.

Plus 2x7's have the added advantage of almost always being lighter.  I just put a 2x7 redfield revolution on my howa mini action and that combo is really nice.  The scope only weights 11 oz.