Generally speaking you get what you pay for with EOs, so if it is cheap, then that is probably because the market has determined that it is of low quality - organic french lavender grown at high altitude can set you back three times as much as lavender grown in less desirable circumstances. Another way to tell a reputable company is that they will list the latin name alongside the common name on the label - for instance, when you buy by peppermint, it needs to be Mentha x piperita, in the US Mentha arvensis is also sold as peppermint (though in the rest of the world we call it cornmint) it is much harsher and contains some less desirable constituents.