The difference in quality results in either more reliability or more features or both. What are the materials used, how much QA/attention to detail is put into assembly, are they inspected appropriately, are they built to standard spec. All of those things affect the price to consumer.
If you shoot a box or two at the range once a year, there is likely no meaningful difference to you between paying $600 or $2000 for an AR.
If you shoot a lot, either via training or competition, or just because it’s fun that’s when you start to see the difference in paying for what you get. Cheaper firearms break more often, or their out-of-spec parts don’t jive well with new parts that are in spec.
There are “cheap” ARs… These are usually priced notably lower than everyone else. If that’s all you can afford, it’s probably fine for low-usage. I don’t shop here, can’t give much advice. Sub $600 today, sub $500 pre-COVID
There are “inexpensive” ARs… Palmetto State Armory (PSA), Ruger AR, Springfield Saint AR, Smith & Wesson M&P Sport, (Colt used to be here), I’m sure I’m forgetting a few others. This is what I recommend to folks who just want an AR, don’t want to spend a ton of money, but don’t want junk. PSA is my fav in this group, but the others are solid too. $600-1000 today, $500-800 pre-COVID
There are “solid, milspec” brands like Bravo Company Manufacturing (BCM), Colt, and Daniel Defense (DD) at the high end. These guys all have military or LEO contracts and literally build the tools “used in the sandbox”. They are literally battle-tested, I recommend these for folks who have the budget and want a super reliable AR that they will likely hand down to their grandkids (assuming they aren’t banned by then). $1000+ today, $800-1500 pre-COVID
There are “high end” brands like Knights Armament (KAC), LWRC, a couple more I forget, I don’t shop here either 
In this price point you get high-end materials and coatings, features like ambidextrousness (sp?) sometimes proprietary parts that perform better than spec. Sometimes better accuracy or less recoil because the whole system is tuned.
There are also a bunch of smaller or boutique retailers/builders that have excellent reputations. Sons of Liberty Gunworks, Centurion, Geissele (priced like high-end, but new to building ARs), Aero Precision, Midwest Industries, Radian, several others I can’t remember.