Oatmeal is cheap and widely available. It contains a specific soluble fiber[0] that has strong evidence for lowering blood cholesterol, and weaker evidence for various other health benefits. It's also very quick and easy to cook in a microwave oven. I eat it every day.

[0] https://en.wikipedia.org/wiki/Oat_beta-glucan

But I’ll add: if you think you don’t like oatmeal because packet or microwave oats aren’t doing it for you, making steel cut oats (not quick oats of any kind) in something like an instant-pot yields crave-worthy oatmeal, IMO.

1:3 ratio of steel cut oats to water, 5 minutes and natural release. I make 150g oats at a time and that yields 4 portions.

Add banana, walnuts/pecans, cinnamon, maple syrup. I wake up excited to eat them every day.

This also means you probably refrigerate some for the next day, which increases the "resistant starch" content, which might make them even healthier for you.

That sounds like a severe glucose overdose.

I’m never liked oatmeal generally prepared in the US. Then I flew to Australia and on quantas they served Muesli which is essentially rolled oats and milk with fruit which was delicious. So it turns out I liked oatmeal just not cooked down into mush. In the states I found bobs red mill which is my standard choice and I get the gluten free one not because I am avoiding gluten it just tastes better than their regular one.

FYI oats are one of those foods that are doused in roundup for efficient desiccation during harvest in the US. No clue if organic oats are dried with equivalent chemical inputs or otherwise concerning processes. After all, leaving oats to slightly mold/mildew isn't exactly great for human health either, but might be the comparable method of choice (for all I know) to shave a few pennies off the per pound harvest costs.

Those foods specifically aren't cheap foods but foods high in soluble and insoluble fiber can be found very cheap. Whole grains are high in insoluble fiber while black beans and oats are high in soluble fiber.

Most veggies are reasonably high in both. Go to a farmer's market if you can afford it. If not, go to a cheap (often ethnic) grocery store.