Generally speaking, you can expect a 2008 Honda Civic SI turbo kit to cost somewhere between $2,150 and $4,250, though you may be able to find cheaper options depending on where you look.
Affordable Turbo Kits, for example, sells a Stage 3 K20 K24 Honda Civic SI Turbo Kit for approximately $2,190.
Vivid Racing, on the other hand, sells the 2008 Honda Civic SI Turbo Specialties HC28B5EU T28 BB Extreme Turbo Kit for roughly $4,248.