cheaper generally means lesser quality, not always but generally. If you are using them for topical applications, I would NOT recommend the NOW brand. I did some comparison testing on the oils available around here and the NOW brand performed the worst! That Aura Casii line from Hedley's is good, but as much as people hate to hear it....Young Living oils are top shelf. I'm not a seller or anything, just a user of oils. They control their product from seed of the plant to sealing your bottle of oil. THATS what I pay for.
It's like organics, you can never be SURE, but it's definitly a BETTER option
Comparison tests I did was on styrofoam (if the oil eats through) paper towel (if it leaves a residue) smell compare and effectiveness (peppermint for headache, which worked/didn't) and lavender (on skin, comparing any irritants. I had about four companies different peppermint and lavender. I do like the YL best. I have had no experience at all with DoTerra.
Just my opinion, hope it helps. And again, I'm not trying to sell anyone oils here, there is enough others out there.