How much does a turbocharger increase a car's acceleration?

I'm looking at a Honda Civic and wondering if I should get EX-T model which has a turbocharger

Depends on the type/brand you put on it. Look it up on the Honda website - don't be lazy, dear. Then you may call up your insurance company to see the price difference for a year's worth of insurance and decide if the extra cost is worth it to you.

By approximately 15-30%, depending on the make and model.

Increase the acceleration as opposed to what? Car makers often put undersized engines in cars which would normally be underpowered and put a turbo on them to make them powerful enough to be acceptable. There are high pressure and low pressure turbos as well. Take for instance the different models of SAABs classic 9-5. The lower priced Linear model has a low pressure turbo and is not particularly powerful compared to the Aero model with a high pressure turbo.

Just because a car has a turbo does not mean that it will be a fast car. Turbos are probably one of the most misunderstood automotive parts of all times.