When you make a custom field searchable, that custom field will display itself in the Advanced Search in the (Leads, Accounts, Contacts, Deals Tab(s)).

Although if it's a number, integer, text or currency field the advanced search for that specific field is an absolute value. Example, I can search for a currency field value of $20.

The Request:
Make an operator before the search value. For example the operator list would include = (equals), > (greater than), < (less than) or between. So the advanced search would be able to handle multiple different search options.