The effect of prayer has been scientifically proved: http://blogs.christianpost.com/biblical-spirituality/scientific-studies-on-prayer-does-prayer-really-work-13400/ (just searched in Google for 'scientific proof on prayer')
And yes, you might have bought the things you eat, but 'someone' or 'something' has provided the source for anything and everything you own: I call it the Universe and Mother Earth, others call it God or Jehovah, or Nature. Everything we have comes from a natural source, wether it's herbs, food, plastic, you name it...
Gratitude and Love have been proved to be the most powerful energies on earth. So why not say 'thank you' to Nature (or whatever you want to name the source), won't do you any harm. I say 'thanks' a lot, whenever I see or experience something that makes me feel happy or grateful, when I feel love for people or animals etc. It has made me feel a lot happier than the 'prayers' in the cult, which felt more like an obligation than something heartfelt.