advanced search box pulldown

hi guys how do I get the advanced search to have pre set pulldown values?

Ie this attached picture is from pistonheads.com and it is the price value.

So i want a user to enter a price at the time of listing – this will be unique ie 12956

The I want the search field to have a max and min price with pulldown boxes ie price upto
1000, 5000, 10000, 20000 – so in this example only the pulldown showing 20000 would show the listing of 12956.

i thought this would be easy so i set up a taxonomy and made the increments. BUT, the problem is when a user goes to add a listing they cannot add a unique value but have to choose one of the increments 1000, 5000, 10000, 20000.

How do I sort this?

cheers