<?php

function is_nearest_price_query( $q ) {
    return isset( $q->query_vars['orderby'] ) && isset( $q->query_vars['price'] ) && 'nearest_price' === $q->query_vars['orderby'];
}

add_filter( 'posts_join', 'nearestprice_join_clauses', 10, 2 );
function nearestprice_join_clauses( $join, $q ) {
    if ( is_nearest_price_query( $q ) ) {
        global $wpdb;
        $join .= " INNER JOIN {$wpdb->postmeta} as price ON {$wpdb->posts}.ID = price.post_id";
    }
    return $join;
}

add_filter( 'posts_where', 'nearestprice_where_clauses', 10, 2 );
function nearestprice_where_clauses( $where, $q ) {
    if ( is_nearest_price_query( $q ) ) {    
        global $wpdb;
        $where .= " AND price.meta_key = '_price'";
    }
    return $where;
}

add_filter( 'posts_orderby', 'nearestprice_orderby_clauses', 10, 2 );
function nearestprice_orderby_clauses( $orderby, $q ) {
    if ( is_nearest_price_query( $q ) ) {
        $orderby = 'nearest_price ASC';
    }
    return $orderby;
}

add_filter( 'posts_fields', 'nearestprice_fields_clauses',10, 2 );
function nearestprice_fields_clauses( $fields, $q ) {
    if ( is_nearest_price_query( $q ) ) {
        $fields .= ', ABS( ABS(price.meta_value) - ' . $q->query_vars['price'] . ') as nearest_price ';
    }
    return $fields;
}

add_filter( 'query_vars', 'price_query_vars' );
function price_query_vars( $vars ) {
    $vars[] = 'price';
    return $vars;
}