<?php
add_filter( 'facetwp_index_row', function ( $params, $class ) {
    if ( 'my_facet' == $params['facet_name'] ) { // change 'my_facet' to facet name
        if ( $params['facet_value'] > 10000 ) {
                $new_params = $params;
                $new_params['facet_value'] = 50000;
                $new_params['facet_display_value'] = 'Less than 50000';
                $class->insert( $new_params ); // insert each value to the database
            }
        }
        if ( $params['facet_value'] > 5000 ) {
                $new_params = $params;
                $new_params['facet_value'] = 10000;
                $new_params['facet_display_value'] = 'Less than 1000';
                $class->insert( $new_params ); // insert each value to the database
            }
        }
        /** more checks for additional values **/
        return false; // don't index original value
    }

    return $params;
}, 10, 2 );