We have a php quote app which works perfectly on one site but is glitchy on another ?

Hi Some time ago we had a php quotation module created for one of out site, so you chose a car, and dates and the system tell the prospective customer the rental rate for the period. We subsequently copied the site different geographical locations, all seemed well. On the original site you chose a small car for 2 days, the quote is £60, on the other sites you do the same and it quotes you for 1 days rather than 2. Everything looks (and is) exactly the same files and content. I need someone to identify the issue and correct please. It only happens for certain vehicle and date combinations, 90% of the configurations show the correct quote amount, very strange. Please help.