If your AR doesn't already have a rail to mount a light and has a regular front sight setup, you can get a rail set for
rail set for the front sight tower for less than $30, plus shipping.
That would let you put a light on there if you don't have one. A full-sized light from a well-established maker of tactical lights (Surefire, Streamlight, etc.) would be the way to go if you can afford it, but if you can't then you can get a
mini-Streamlight with a plastic rail mount for $35, and it is bright enough for target ID at HD distances; also makes a nice backup to a bigger/brighter light. (I use that light as an EDC pocket light and love it, but I don't own the rail mount for it yet.)
Here's a good
Vickers-style quick-adjust sling for $40 plus shipping.
BCM
Gunfighter Mod 4 (medium) charging handle. Well worth the $45 plus shipping.
Here's my mostly-on-the-cheap AR with many of the above (the front sight rail, sling, and charging handle).
Edited: See post below for pics that actually show up.
That rifle also wears a
Magpul MBUS rear sight, currently $55-ish.
This isn't a mod, but here's a
good book on carbine shooting skills for $30, and shooting skills are more important than gear...or if you are more visually inclined, here is a
Magpul DVD set for $34. A carbine class would be better, but you can't get into a carbine class for under $50.
These aren't mods either, but if you don't know what gun oil to use on the bolt and bolt carrier, I am a huge fan of
this. Seriously. Just don't put it on the outside of the gun, because it stays wet and stays put (and stick with full synthetic; non-synthetic motor oils smell pretty bad when they get hot, and don't lubricate as well). For quick cleaning,
one of these in .22 caliber, and a can of
this.
Finally, have someone with the proper staking fixture torque and stake your gas key screws if they're not already done properly, and make sure the castle nut is properly staked as well. You can probably find a reputable armorer to do them both for well under $50.