Someone posted a link to a Colt LE6940 for about $950 - if you did a search I bet you could find it.
If you ever plan to take a class, go with the Colt. BCM is also a good option, but with what Colts are selling for these days they are pretty close on cost.
No you can't build an AR as reliable as a Colt for less money. You can build less for less though, pretty easy.
As far as the Sig goes, if it priced really right it would be ok for recreational use. They are nothing special though, and if it is within a couple hundred dollars of the Colt/BCM option above it would be a stretch to say it is worth it.
Put one together yourself with cheap parts, it will be just as good as the Colt,BCM,DD,LMT
Better yet, go root through the dumpster at the state fair grounds after the next 1500 and throw together an AR with the leftover parts in there. It will be just as reliable.
I was able to save a lot of money by buying used. For me it wasn't the brand as much as the condition. I found a slightly used Rock River, 1 in 9 twist, with quad rails in mint condition. The $300 I saved purchased a Burris AR 332. I added a cheap Nebo light, a Grip-Pod and a magpul multi purpose sling which are all the toys I need. It works well for me at a cost just over $1200. There are a lot of slightly used ARs on the market and I wouldn't hesitate to do it again.