Okay, I looked at the 2017 Chevy Spark, LS, priced at $10,599. Not a bad price.  Not so fast.  The original sticker was $13,000.  That's only a 20% drop in price from getting a new car.  

So the question is what is the average deprecation costs on a new car, any new car, regardless of year, regardless of make, regardless of anything?  

Depreciation really is determined by make, model, and year.  See this chart on a few cars.  Note how with some the depreciation is 20% while for other cars you're looking at 25% to 30% or more.  

According to bankrate, these are the five cars with the fastest depreciation rates:

2012 Kia Sedona, LX: $24,900.  Expected depreciation in first five years: $17,730.

2013 Chevrolet Impala, LT: $27,385.  Expected depreciation in first 5 years: $17,983.

2012 Jeep Liberty Limited Sport 2WD: $23,395.  Expected depreciation in first 5 years: $15,239.

2012 Suzuki Grand Vitara Premium RWD:  $22,349.  Expected depreciation in first 5 years: $14, 379.

2013 Jaguar XJ AWD: $76,700.  Expected depreciation in first 5 years: $52,014

Also, it looks like depreciation rates are calculated by years of use due to business use and not simply some arbitrary percentage.  See here: 

the following table prescribes these limits.

Car depreciation limits

Depreciation limitations for passenger automobiles (that are not trucks or vans) placed in service by the taxpayer in calendar year 2009, for which the 50 percent additional first year depreciation deduction applies

Tax year	Amount
1st tax year	$10,960
2nd tax year	$4,800
3rd tax year	$2,850
Each succeeding year	$1,775

Your maximum deduction under this table is $10,960, reduced by your personal use percentage. If the car is used 20 percent for personal purposes, then your maximum deduction for 2009 is $8,768 and in the first three years totals $14,888. Because the car’s business basis at 20-percent personal use is $88,000, you can see that it will take several decades to recover the car’s cost.  Even CarMax points out that cars lose 20% of their value the first year. $10,599 of $13,000 is not even 20% of the original sticker.  20% would be $2,600 from the sticker, around $10,400.  

That same Carmax article made a great point:

At the turn of the millennium, for example, new cars were being rolled out at a rate that far exceeded demand, so leasing became popular. As leases lapsed, the used car market swelled, and with it the depreciation used car buyers faced. In other words, cars depreciated more quickly because the demand for used cars was low. Things leveled off for a while, but the recession of 2008 sent depreciation rates soaring once again.

Psychology does play a role in value: 

Beyond market forces you can’t control, psychology also has a role to play in how much your car’s value drops year to year. Cars that are popular with the public fare much better than those deemed less desirable.  

Just when I am looking for a formula, something to count on, I read this: 

So don’t expect your used car to depreciate at a consistent level. A lot can change.  Oh, boy!  

Miles count when depreciating a car. 

The number of miles you drive will also factor in. The fewer miles on the car, the better your odds of trimming your depreciation. A good rule of thumb is 10,000 miles per year. That might seem like a strict limit if you have a long commute or live in an area where you have to drive everywhere you go, but the lower the mileage on the car, the more it’s going to be worth.  

Color too.

Finally, consider a neutral color. If you’re really worried about depreciation, think twice before you buy that used Charger in “Dukes of Hazard” orange. You may like it, but when it comes time to sell, you’re going to significantly reduce the number of folks willing to buy it.  

Reputation is also a factor.  Like, safety.

cars with strong safety features will hold their value better over the long haul.  

Too many things to calculate.