PHP craigslist clone modifications

Hi, I am using a craigslist clone ([login to view URL]) and then developers have fallen off the planet. I would like the code modified so that I can request payment for a specific category like jobs. The code is PHP5 and mySQL database. There is an admin back-end. that allows you to add catergories - here we would like a field enabled and for $ amount. Then when posting an ad this $ amount must be paid for. Currently it does allow a field extended ad to be paid for, but we would like it if enabled then take the $ amount. A sample site also is at : [login to view URL];cityid=1&lang=en&catid=5&subcatid=42 and you can see the form posting. Pleaes let me know if you need further information and please let me know how long it will take and your estimated price.