SELECT id
FROM products_price 
WHERE round_price(price * currency_rate(cid)) BETWEEN 100 AND 20000

EXPLAIN
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	prf_vdp_store_products_price	ALL	NULL	NULL	NULL	NULL	10963	Using where

CREATE DEFINER=`root`@`localhost` FUNCTION `currency_rate`(`cid` INT)
	RETURNS DOUBLE
	LANGUAGE SQL
	NOT DETERMINISTIC
	CONTAINS SQL
	SQL SECURITY DEFINER
	COMMENT ''
BEGIN
SET @convert_cid = (SELECT value FROM configure WHERE `var_name` = '_currency_id' LIMIT 1);
RETURN 
(SELECT rate FROM currency_rate WHERE 
	main_cid = cid 
	AND convert_cid = @convert_cid
);
END

CREATE DEFINER=`root`@`localhost` FUNCTION `round_price`(`price` DOUBLE)
	RETURNS double
	LANGUAGE SQL
	NOT DETERMINISTIC
	CONTAINS SQL
	SQL SECURITY DEFINER
	COMMENT ''
BEGIN
SET @cid = (SELECT `value` FROM configure WHERE `var_name` = '_currency_id' LIMIT 1);
SET @precision = (SELECT `round_precision` FROM currency WHERE `id` = @cid LIMIT 1);
SET @coeff = POW(10,@precision);
RETURN CEIL(price*@coeff)/@coeff;
END

CREATE TABLE `products_price` (
	`id` INT(10) NOT NULL AUTO_INCREMENT COMMENT 'Product ID',
	`pgid` INT(10) NOT NULL COMMENT 'Product Group ID',
	`cid` INT(10) NULL DEFAULT '1' COMMENT 'Currency ID',
	`supplier_id` INT(10) NULL DEFAULT NULL COMMENT 'Supplier ID',
	`date` TIMESTAMP NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP COMMENT 'Date set',
	`price` DECIMAL(15,2) NULL DEFAULT '0.00' COMMENT 'Price',
	PRIMARY KEY (`id`),
	INDEX `pgid` (`pgid`)
)
COLLATE='utf8_general_ci'
ENGINE=MyISAM
AUTO_INCREMENT=10964;

...BETWEEN 100 AND 20000
/*Return 5000 Row*/

...BETWEEN 19000 AND 20000
/*Return 50 Row*/

SET @cid = (SELECT `value` FROM configure WHERE `var_name` = '_currency_id' LIMIT 1);
SET @precision = (SELECT `round_precision` FROM currency WHERE `id` = @cid LIMIT 1);
SET @coeff = POW(10,@precision);

SELECT pr.id
FROM products_price AS pr
LEFT JOIN currency_rate AS cur ON main_cid = pr.cid AND convert_cid = 2
WHERE price * cur.rate BETWEEN 20 AND 20000

SELECT pr.id
FROM products_price AS pr
LEFT JOIN currency_rate AS cur ON cur.main_cid = pr.cid AND convert_cid = 2
LEFT JOIN currency AS c ON c.id = cur.convert_cid
WHERE CEIL(price * cur.rate*POW(10,c.round_precision))/POW(10,c.round_precision) BETWEEN 20 AND 20000