For long range, I chose an SII 6.5-20x50 for $650 plus an additional $50 mail in rebate. I kicked around a large number of scopes, and Sightron was my final choice and I stand behind it 100% (and no, I dont work for Sightron, just like their stuff).
One scope you might consider is the Super Sniper scopes by SWFA.
SWFA Riflescopes SWFA SS Rifle Scopes SWFA SS Scopes
A 16x42 with Mil-dot, rear focus, and good for use on a .50 BMG for $319.00. As someone else said, a fixed power is good for just shooting at the range. If that is what you are doing, stay fixed power, save some cash for other upgrades.
Also, might try a Nikon on for size. They have good glass and are reasonably priced.
Good luck with the rifle. I just got my first one set up (REM 700 SPS Tactical .308, Harris style bi-pod, Warne tactical rings and 20 moa base (for free as I won them in another shooting sport) and the sightron 6.5-20x50 SII). I also am looking at range time, but want another use for a rifle like this.
If I had not won the Warne Tactical stuff, I would have used Weaver bases and rings. They are inexpensive and I have NEVER had one fail on anything from .223 to .308. So if savings is your game, you might look at those.
All in all, I have right at $1200 wrapped up into it and it shoots sub 1/2" groups at 100 yards.