It amazes me that not everyone has cottoned on to the wonders of this cheap, versatile supermarket buy. Eucalyptus, tea tree and lavender oil are all super cheap, potent, easily available, NATURAL and multi-use.

If you only choose one oil - eucalyptus is your friend. I have a bottle that has lasted 6 months. I use eucalytus oil:

to dab into my nostrils to prevent inflammation from dust pollens and cold/flu symptoms
added to saline solution for a nasal spray
dabbed on blemishes to reduce inflammation and blitz spots
added to floor cleaner
to wipe down bathroom surfaces
to wash gym clothing - out, out damn germs!
diluted as a spray for the yoga mat. Trust me.

Then there are two more that will go the distance. They have multiple uses and because they are so potent, you'll use only a tiny amount at a time. This is going to be magic for your budget and your health. I use lavender oil:

rubbed gently on my temples to relax
spritzed on my pillow and bedding before sleep to de-stres
added to boiling water for a facial steam (just hold a towel over your head!)
added to fragrance-free, basic moisturisers for scent and anti-inflammatory properties
massage oil for the muscles

Finally, though the smell takes me back to my teenage years, teatree oil dabbed on spots will blitz them. It's also great for cuts, abrasions and also for cleaning duties.

It's worth buying 3 at a time and saving around 15% - I order online at Bosisto's but these are also easily available at chemists, pharmacies and supermarkets.