Black plastic piece on the left curverail for timing chain does it affect chain?

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Hello, I live in the Bronx, and to my horror with no prior problems my 2003 toyota corolla S turned off with some smoke from the engine on the expressway on my way to Suffolk County for a visit. Me not knowing LOng Island, raining and still not close to my destination I had to call a tow truck, it was after 7:30pm on a Saturday I know it would cost me an arm and a leg to tow it back to the Bronx. So I ask him to take me to the nearest car repair shop in the area and near a train station. The mechanic determined that it sounded like the timing chain but that he wouldn't now until he goes in. Well being that it was late that he would call me on Monday. When the Mechanic calls he tells me that a bolt broke on the camshaft and that there was nothing wrong with the chain but that he had to replace the camshaft, five valves and head gasket,for a total of 1900. A few days later he calls me and tells me that there was another problem that wasn't noticed until they put everything together and turned on the car. On the left rail there's a plastic piece missing and that was the problem. And now he's saying that he knows that he quoted me a price and that he would only charge me the wholesale price for the part which is about another 340.more. Can that piece be brought separately,can the rails be brought separately or do they have to buy the whole timing chain kit? In reality I'm sure am taken advantage of... Thank you No choice