I don't believe there is such a thing as pure calendula oil unless it is an essential oil (which I would NOT put undiluted on skin!) Calendula essential oil will run about $40 an oounce or more, and would require diluting.
Most calendula oils I've seen are "infused" oils, which means that the calendula flowers are steeped in oil for a period of time to incorporate the plant's properties into the oil. There are good and bad quality infused oils out there, and some use more of the plant than others. I perfer to make my own calendula oil, and my infused oil is an amazing deep orange, much more potent that some of the pale yellow infused oils I've found on the market. There are some reliable brands out there. I believe Herb Pharm makes a nice therapeutic quality infused calendula oil.
Calendula can certainly help with skin issues, but if the eczema is being caused by deeper issues than the calendual may only help with some of the superficial healing and soothing, but will not eliminate the cause of the eczema.