More expensive receivers usually offer the following:
Better remote
More features (you may desire or not)
Bigger beefier power amplifiers
Possibly better sound.
More channels (9.1 or 11.1 versus 5.1 or 7.1 etc)
It depends what you want to do with it. People like to buy the big expensive receiver and often never use the added features or never needed the power.
What I would look for is the least expensive receiver you can find that offers the surround features you need and channels you need (and possibly one with preouts so you can add power amps.
Receivers are basically disposable toasters with horrid resale value. Stereo shops are littered with $3000 receivers 8-10 years ago that sell for $200 now.
Once the feature set is out of date they're pretty much worthless. I would treat a receiver like a laptop - in three years they're pretty much obsolete junk.
So if you buy a $700 current just released receiver you will pretty much get 90% of the features you would get from a $2100 receiver. If you run reasonably efficient speakers it's doubtful you'll notice an audible difference (in the same line - comparing say a Marantz to a Marantz or Deonon to Denon etc). Then in 3 years you buy another $700 receiver - chances are that new $700 receiver will blow the current $2000 receiver away in terms of features. But even if it's "just on par" you have now equaled the $2100 receiver and over 6 years you've spent $1400. Plus three years after that if you spend $700 you can be pretty much guaranteed that any $700 receiver in 2018 will beat any current $2100 receiver in terms of remote feature set and perhaps power and sound as well all for the same money spread over three purchases.
Further you were able to sell the previous 2 receivers or use them in second and third systems. So over 9 years you have three amplifiers. Or one receiver that had to last 9 years (buying one now for 9 years or buying one now - for 3 years, another for 3 more years, and another for 3 more years). The $2100 one will be out of date in 3-6 years. But with the three receivers - you will always be "up to date" on technology and in year 9 when or if they've replaced HDMI with something else your machine will have the connections or if they're up to 20.2 you're machine will have the connections.
As for the power amp section - if the receiver has the preouts you could buy yourself some good well built power amps that probably beat any in the receivers and you can cart them along to every receiver in the future or buy surround processors instead.
A Receiver is like an all in one printer. It does everything but nothing particularly well. Their reason for being is features. But if the features are passe every few years you may as well go as cheap as possible with all the features you require and treat them as disposable.
Further if the $700 one blows up after the warranty it is easier to swallow than the $2,100 or $5,000 beastie if it fails.
Receivers have very high failure rates. I worked at McDonalds - we went through 3 receivers in a year - Yamaha, Denon, and Sony and they only ran two speakers. I had the flagship Pioneer Elite in the mid 95 - (the first one was DOA) - the second one was okay but my current 2003 $350 Marantz sounds every bit as good (in some ways better) and is far more advanced in every way. 8 year later a $350 receiver embarrasses an over $2,000 receiver.
Frankly I would look at something like this.
http://www.crutchfield.com/S-guT4TWbv7mV/p_642NR1603/Marantz
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