2003 Toyota Corolla Oil Tank

O.K, I'm not sure if I'm describing things here accurately, but my car's been leaking oil for several months now. I've brought it in twice to a garage (who I do trust and have worked with for a while now). They replaced the oil cap/gasket (I think that's right) for a few bucks each time, thinking it was that. Unfortunately, the problem persisted and now they're thinking they need to replace the entire oil tank/reservoir (I'm not sure I'm calling it the right name) due to a possible hairline fracture somewhere with the tank. They said it would cost around $400. While I know I've most likely called everything by the wrong name, from best you can tell, does that sound like a reasonable price? Any information you can provide would be much appreciated. Thanks for your time.