I'm looking for an air rifle to use for casual practice when the weather is bad. It needs to be accurate, but doesn't need to be high-powered. I'm going to shoot it in the basement when I can't get out to the range. I'd like to keep it in the $500 or below price range. Suggestions?

You can get lots of good guns with $500.
Especially if you look for a good used one.
Try look at a few guns if poss to see what sort you like.

Anything of use outdoors at 30 or so meters is really too much for a basement range unless you got a BIG basement, maybe get a little gun or even an air pistol for indoor plinking.

http://www.airgunsofarizona.com/usedguns.html

the R9 might be nice

Dunno if I've mentioned this before, but about 25 years appx ago I bought a Sharp Innova pump up, made in Japan. The wood was soft crap, it was difficult to get a pellet in, an absolute pain to pump up to the max of about 10 pumps.
And over 3pumps, boy is it noisy. Came with a peep sight, takes any scope,(no recoil) and fixed barrel= better accuracy, esp with scope.

But, in my house, mostly in the garage, its come into its own. One pump is plenty up to 10 yards. 2 pumps no problems.
Less power = less backstop needed.

Accuracy is terrific. At 1 pump trigger is quite light. It gets heaver with each pump.
Appart from testing pellet accuracy, I use it off hand as practise for our range shoots.

And best for last, each pump lifts the point of aim a little, but dead in line.

Get a AirForce Talon SS ..... Super Accurate and lots of upgrades.

I have a RWS model 36 that is very accurate. it was less than $240. I bought a variable 3x9 rws scope second hand. no regrets. gotta be ten years old now.

Beeman R7

Great trigger, quiet, and extremely accurate.

I have a Steriod from www.mac1airgun.com for my basement low power shooting. The peep is very nice. The trigger is excellent. I could pump it up if I wanted to go outside.

I also have a PCP, but that is out of your price range.