It depends on the quality of essential oil that you use. I use most of my essential oils everyday (and up to four of them at a time) undiluted as they are guaranteed therapeutic grade. There is a list of essential oils that you can use undiluted but depends if you can be sure of the quality of the oil. You said "preferably not too expensive". I have been using essential oils for over 25 years and it is only when I changed to the (expensive) therapeutic grade essential oils that I could use them everyday. You get what you pay for! There are so many added benefits (ie feeling good!) if you do use good quality oil.