Here's some of my own work (Write-up). I saved this when I was helping a buddy build one of his own:

Just to let you know, the rails do not come cheap. You can pay about $100 for some sub par ones. You'll find good ones in the $200-$300+ range. Plus these rails will be made of aluminum or steel, so they will add a good bit of weight.

The only other handguards that i'd recommend on the cheap are the Magpul MOE Handguards, which can be found on their website. The Guards run at about $35 and then you'll have to purchase the rails for another $6-$9 per rail. Plus they're polymer, like a glock, so it will save a bunch on the weight issue. I've never personally held one but I know guys on the forum that LOVE them. If I was doing a lightweight build, I would probably take a shot at those MOE guards.

Ok, you'll have to figure out if you want a Mid-Length AKA 'Middy' (16" barrel+flash suppressor=17"-18"; and you can swap the stock A2 flash suppressor with another type/brand by screwing on and off) OR do you want the M4 style which the barrel length is 14.5" plus the flash suppressor WELDED on (therefore you can't swap easily) to reach a total length of 16".

By law, you can't own a rifle under a total barrel length of 16" unless you fill out some paperwork and pay a $200 tax for a 'SBR'/Short Barrelled Rifle stamp.

I bought my upper reciever complete because I figured i'm not really saving a whole lot on that part. The only thing I swapped on my upper was the dust cover and I plan on changing my flash hider to a Vortex soon. BCM, Noveske and Spikes Tactical offer great complete uppers.

Personally, I prefer the Mid-Length. There have been tests done that are found on the internet that show the Middy is more efficent on the internal componets of the rifle. Meaning, a Middy will last you longer before you have to replace parts. How much longer? Well a barrel will last close to 20,000 rounds before you actually notice any accuracy issues in either rifle. But, the BCG will probably fail before then and tests show that it fails faster in the M4 style.

Middy's are just a tad bit more accurate given the extra length. Also the easy swap of the flash suppressor, which I also prefer the Yankee Hill 5C2 comp or the Smith Enterprise Vortex flash hider. Unless I was actually using the rifle as a SHTF/tactical killing machine, in which I would pay the tax for a 12.5" barrel, i'd go with a Middy.

It's kinda the same for the lower reciever as to whether you'll want to build. You can buy complete or incomplete lowers. This time I decided to build my lower. The trigger group is one of the most important factors in shooting so that's why I decided to build. There are plenty of different sytles to choose from, including different stages of triggers. Some are very light like a Timney trigger (less than 3lbs of pulling force), that could run you close to $300.

One day i'd like to try one of those, but for now I settle for a custom setup, in which the guy/website I bought it from takes the quality components from different brands and puts them togethter. I think my trigger group costs me about $80, which it's a great setup, very crisp and i'm accurate with it. I think my dad installed a Rock River Arms stage 2 setup in his before he passed away. I don't believe I have shot his setup yet.

Installing the trigger group is very simple as I just followed a couple videos from YouTube. With the right tools and having all the parts, it might take you about an hour to complete.

Here's the starting point on what you need to decide on the upper and lower reciever standpoint of things. Some of the parts like BCG, Stock, Grip, are your choices, but i'll give you some recommendations later if you want.

BASIC AR-15 SETUP:

Upper Assembly:
BCM BFH 16" Middy Complete Upper w/o handguards - $489
Magpul MOE Handguards - $35
Magpul MOE Rails - $15 to $25
BCM Charging Handle - $45
BCM BCG (Auto) - $140
Troy Industries BUIS (Back-up iron sight) - $120 (You can find cheaper, this is just what I run with)

Approx. $849 in upper

Lower Assembly:
Spikes Tactical Stripped Lower - $99 (This part brings a wide range in the money aspect. I have a Yankee Hill Machined lower that ran me about $140. I hear Spikes has some nice quality products, so for a budget build it's not a bad choice.)
G&R Tactical LPK; Lower Parts Kit (Trigger assembly w/ Magpul MOE trigger guard) - $94
Basic Pistol Grip - $4 (Easy upgradable part, I went with a Houge grip for $18, but you'll need a grip with that LPK package)
Complete M4 stock from BCM - $83 (This can raise the price of you rifle very fast, the one I listed is just basic with all the parts included; buffer, buffer tube, spring, castle nut, endplate, stock. I have a Magpul ACS stock which costs $130 alone without the other parts. So I have roughly $186 in my stock setup alone. Just something to think about.)

Approx. $280 in lower

Approx. total cost of basic rifle with iron sight setup: $1129

What I have listed are great quality parts, but it can still get more expensive than that. For instance, the better rails will def raise that total even higher by about $100-$300 depending on the rails. The stock can increase price as well as stated above. The LPK can go either way, for high-end you're looking at $200+, for cheaper side $50-70. The stripped lower reciever is really up to what roll stamp design you want as prices can range from $100-$200.

Well this is a look at a quality 'middle of the road' stock build with iron sights, ready to shoot.