You can generally buy cheaper, quicker than assemble. You can definitely buy parts that are likely used dirt cheap, and beat most retailers, but - you get what you pay for.
$400 AR's have been done. In building, many upgrade this or that, and keeping it under $900 can be tough. All out, you could blow $1500 easy, and it really doesn't do anything much better than a $1100 gun - because the factory starts getting it's discounts for volume buying into the picture. They pay less than $45 for stripped lowers or uppers when they buy hundreds. Even if you get a nice blem for $48, you add shipping and handling, and do that too much, you will eat up a lot of the budget.
Your best buys are getting kits to assemble major groups to ensure you get all the little parts, and reduce S&H to a minimum. Nobody likes an $8 charge for a $2.00 gas tube roll pin - and you won't find many parts at an auto supply or hardware store, gun designers deliberately choose obscure sizes and thread pitches to shut them out.
Shop to get the best price shipped to your front door. Expect some items to be commodities - meaning you can practically throw a dart at a list of choices and you'd get the same quality and functioning part. Lowers are a great example, there's less than a dozen forging them, another two dozen more machine them only (including Colt,) and they actually make all the different roll marks to contract. Elmar's Bait and Beer in Okmulgee isn't doing the work themselves - but has good pricing and works.
Your best route is to decide what it specificaly will be used for, and then pick the best caliber, barrel, upper, optic, furniture, and trigger that support the use. Doing so cuts down the apparently unlimited choices to the vendors that actually make parts for the specific use, and keeps from building a franekngun that resembles a cross from a dachshund and shepherd. Lots of CQB 14.5" sniper guns out there, they don't do either job well.
Gabe Suarez is recently noted for saying any AR is a specific one job gun, not a universal Gun To Rule Them All. Obviously a 600m precision rifle doesn't share much else than the upper and lower compared to a door kicker on a SERT team. That is entirely the point, but a lot of builders miss it gathering up the latest assortment of state of the art cool parts, many of which have nothing to do with mutually supporting the same use. They wind up being a snapshot of the market, not actually a using gun - which is why you hear so much about changing things. Somebody actually hasn't done their homework and sorted out what they really wanted to do with it.
Your best bet is to know what it is you want to get, and then be ready to jump on a sale or bargain with ready cash. $79 barrels, free lowers with uppers, a fire sale introductory price, or finding one of the last 20 blem name brand forged milspec uppers for a song can't be waffled on for days. Some of these deals last just a few hours, and you have to check daily for them - not just expect to be told. There's some surfing involved, and arfcom at least keeps a thread on the bargains available.
Read the construction stickies, there are a lot of tips, and buying armorer's tools for some of it is expensive and completely unnecessary for parts of the job. They are really for controlled disassembly - and on the first build, you don't need them. Vice grips and old drill bits will assemble a lower with less scratches and oops than hammers and punches you have to order from gunsmithing suppliers. A special vice block and barrel wrench is nice, but a set of jaw inserts and a 12" pair of Channelocks or pipe wrench you already have can git 'er done. If you plan to hunt and shoot it, it'll get marked up anyway - and safe queen builders don't really ever see their "investment" return for them anyway. They put it together - which makes it another kitchen table gun.
Have fun, it's an interesting hobby, and don't be surprised to find you're thinking about doing it again before you even finish the first one.