what are they exactly?
how much bhp do they add?
and lastly how much do they cost fitted?
All petrols have them (ok the new BMW stuff doens;'t as that valvtronic etc)
They limit the air flow to the engine so limit power.
When people refere to cars have throttle bodies they mean the car has one per cylinder since the single one isn't as efficient at letting air in ie making power.
Best do a search though for pictures and what they are.
Gains depend on loads of factors what engine you stanrt with what sie bodies other bith the car has etc.
Cost depend if there kits out there say £3k for a car which has a kit out there for a good setup and stand aloone ECU etc. You can buy bodies themselves quite cheap its the mapping and ECU and the piping and injectors and fitting and manifold where the cost realty adds up.