Add chained select boxes search to existing website

I would like to alter existing bespoke real estate software package (realtyprolite) so that there is a "chained select box" search with three categories of state, city and town (see holidaylettings'co'uk/[login to view URL]). My site is already set up for these but with the software's existing "show all" search facility. So if I enter California it still shows all cities and towns, whereas I only want it to show cities in California then if San Francisco is selected only towns in San Francisco area to show. The existing databases for state, city and town are mutually exclusive and would need an additional sub-table to link them-probably using the city to get a state/city and city/town indexing. Also want a property owner's private login where they can add or edit their property (currently this can only be done by me), with the session cookies so I can vet before allowing to be processsed. Please can you tell me the cost would be split between the 2 tasks.