Many factors can affect the installation cost of a 22kW standby generator. Local codes vary and every installation is unique. An exact answer is impossible give all the factors to take into consideration. A simple installation allows generator placement very close to the natural gas supply and to the electric meter and main circuit breaker panel. Move the generator away from either utility and the cost increases, especially if the installation requires outdoor watertight conduit or multiple transfer switches. Additional plumbing for the gas lines can increase costs substantially. Old electrical panels may need upgrade before the installation which can also increase the cost.
On average, the cost to install an air-cooled 22kW generator be similar to the cost of the generator itself.  Serve as the general contractor on the job and do some of the work yourself and you can substantially lower the generator install cost.