You can buy a two stage from Rock River for under $100, I have these on two ARs and am very happy with them.
The price goes up from there. JP and Jard make a great adjustable trigger for around $115.00. The Chip McCormick triggers are very nice, they are around $145.00.
There are some others out there but you should be able to find one that fits your needs from the triggers listed above. I saw something on Gun Broker in the Ar-15 section that listed a new trigger set up for around $35-40, not sure if it is new springs, hammer or what.
Timney is also making an AR trigger now - not sure if it's adjustable or not; it runs close to $200. Jewell and Grieselle (SP?) are probably the best two available; Jewell is right around $210 while Grieselle is close to $300. For the money, the RRA is hard to beat, especially if you have it tuned by White Oak Precision before you install it; still only about $130.
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