I also offer organic foods and my rates reflect that. However, I make it clear to my parents (and have raised my rates based on organic prices in the past) that my grocery expenses require me to charge more than others in my area.
I've found that in the last 7 years of being licensed, this was a huge selling point for me (I also use organic cleaning supplies).
BUT, I do not purchase ALL organic foods. Definitely organic milk and all the "dirty dozen" foods. Then, I use my best judgement when it comes to canned, dry goods and cheeses. Depending on what I'm using it for. I also bake most of my breads.
All that (and a food program) keeps my expenses more manageable.
But, I am in Northern US, so I'm sure your expenses differ from mine!

As you get more seasoned with how to deal with parent issues, you will grow more bold in dealing with people who won't adhere to your policies. I've found that just being honest is the best approach. If you lose parents because of that... well, they weren't worthy of you!