Instead of "hacking" something by pouring more things in, try layering, e.g. do the "hack" directly, by applying the fragrance + whatever you choose to add, right on your skin. Dilute any essential oil in some sort of carrier oil, because some can be really irritating at full strength.
Many people layer even high quality fragrances in order to obtain a specific result; it has the advantage of allowing you to experiment with infinite combinations.
I've never tried "hacking" a fragrance in the way you describe, but if there are fundamental problems with something, you can't really cover it up, regardless of whether it is of a poor quality or just doesn't work with your chemistry.
Price is no guarantee of quality, but in general the unusually/suspiciously cheap are not a great bet, and the most carefully crafted offerings tend to run expensive.
Ultra-cheap essential oils are seldom any good; find a reputable firm and stick with them. If your budget is small, go with less expensive selections (e.g. citruses, certain neutral florals such as lavender) rather than low-end versions of things that--when authentic and of good quality--cost the equivalent of an organ transplant.
As for expensive failures... well, try RE-Baying them
Finally: Unless you like having collections of amusingly bad knock-off bottles containing unwearable juice, buy from sellers with good reputations (there is a thread for this on Basenotes); also, try before you buy, you can order samples from luckyscent (3 US$ a pop) or the like... you needn't buy from the place where you did your testing, after all.