Rice, oats, dried beans, and lentils are among the cheapest calories in any grocery store. The expensive part of plant-based eating is the packaged stuff, which is also the part you need least.

Extracted seed oils — including olive oil — are the thing I've cut hardest since the T. Colin Campbell certification. Whole nuts and seeds are a different story and stay on my plate. Indeed it's a tough learning curve when you've always sautéed veggies with oil. Start subbing out cooking food with oil (using vegetable broth) and you'll notice a significant difference.