I am a bit confused here on what you are actually asking but here goes....
if you want to store the range values on the database and you are having the users enter numeric data then it only seems sensible to store the fields in a numeric format. Since the data is a price I would tend to store it in an integer field in pence (GBP) or cents (USA).
If you are asking what is the best way to ask for the field on the screen, then I would just use a text box unless you really want to limit the choices in which case a select box could be used (and the data could be stored as an enum).
You don't really say what and how you are then using these values for with regards to the searching you mention but I assume if searching a database you know to build an sql statement along the lines of
$query = "select product from table where '$low_range' <= price and $high_range' >= price";