What is the cost to replace the Drive Belt Tensioner , My vehicle all locked

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and didn't want to turn the wheel, My mechanic said it was the Drive Belt tensioner that burned the belt and broke off, he said the belts were like 19.00 and 19.50 but the Tensioner replacement part was $470.00 I believe this is way to much, not sure can you tell me what is the estimate for this belt tensioner part..... Just paid 2 months ago 1800.00 due to timing belt and Valves bent....It still turns on and all electrical works I didn't drive it pass when it happened just towed it...