On average, how much does the value of a car model decrease by each passing year?

Take a honda cr-z for example, it costs 20k now. In- say- 6 years, approximately how much will it cost?

It depends on a lot of factors
The lower the miles it clocked, the lower the rate of depreciation. The better the condition, the higher it's value.
The car's name also plays a role. Reputable brands like Honda and Toyota hold their value a lot better than a Dodge or Chevrolet in similar year/condition.

Depreciation is different year-to-year. It's highest the first year, then lower each year afterwards. An average car depreciates 50% in 3 years.