Here is a great Leupold scope that slots in right at your budget (mine was $399) and is perfect for what you want -- the 1.5x magnification allows you to shoot up close just like a red dot version (such as an Aimpoint or EOTech) - you can even shoot with both eyes open for better situational awareness and fast target acquisition, especially on movers [trust me -- it works better than you think it would, once you get used to it :wink: ], and it is a variable scope that can be set at 2, 3, 4 and 5x, depending on your choice/preference/target size, so when you want to go to the old tried-and-true one eye closed long range shot, it is more than up to the task:
http://www.leupold.com/hunting-and-shoo ... -5-5x20mm/
It is an extremely good scope for the money. I own other scopes that cost a lot more, but for the money, and for your intended use, this Leupold will be very hard to beat. Here's a picture of it on my SIG 556:
You will still need to buy scope rings (the scope in the link and picture has a one inch tube) which will run you anywhere from <$20 (for the scope rings you see on the scope in the picture) to ~$65 (for the new Leupold scope rings I bought for it [today, actually] as replacements for the original Springfield Armory set I had on hand in the right size when I first bought the scope).