I agree with smkit. The cheapest oil for cutting board is the mineral oil from pharmacy. You can pay $5 and get 32 oz:
Do not use baby oil. Although baby oil is mostly made of mineral oil, it also has fragrance and is not considered food safe.
You can get mareket cutting board oils and they are essentially mineral oil and usually have additional ingredients like lemon oil or beeswax or tung oil or vitamin E.
Go to a pharmacy and ask for mineral oil. It is used for digestion, so if you can drink it, it is food safe. I've seen IKEA and Bed Bath & Beyond also have it for a reasonable price. Or you can order it online. A lot of people use Boos Mystery Oil. That will run you about $10.