It depends on what the requirements are, the operation process and whether it fits into the existing infrastructure, vendor support etc. E.g. is it purely a ticket entry system or also a vending facility? Is it a system that sits on PC or is networked to entry gates etc...
Also, as I'm sure you realise, it has to be sufficiently tested as I'm sure if it goes wrong there will be some mightly pee'd off people, and that will add to the costs.