Assuming the vehicle requires no welding an underbody restoration would typically involve the following steps and will justify the price.
Removal of all underbody treys, including ancillary components such as fuel and brake lines, exhaust, heat shields, suspension, electrics etc.. and ideally the engine.
All the aforementioned components and the underbody of the car is then media/vapour blasted to remove all rust
Next the underbody is epoxy chassis coated and joint sealers refreshed.
The body and parts will then have factory style coatings applied + along with any colours required.
Once dry all can be coated in A clear protection coat prior to being reassembly with new bushes, joints, fuel and brake, plastic under treys, hoses and fixings and fastenings.
As you can imagine from the steps above labour can easily exceed 140hrs to perform an underbody restoration.
Underbody restoration is likely to set you back circa £7k-£12k + parts