Don't be a sheep and buy it when your dentist tells you that you need a crown instead of a filling or onlay because your tooth is too weak or a filling would be too big .Fillings and onlays never make your tooth weaker or more succeptible to fracture , unless the onlays or fillings contain mercury . The overall principle is that your tooth will be healthier and will last longest, hopefully your lifetime , if the least amount of healthy tooth structure is removed.If your teeth are already stubs because you are an alcoholic you might require a crown . Otherwise I would tell the dentist that I can't afford a crown and therefore I want a filling for now to avoid an argument or getting thrown out of the office as happened to me several times when I argued with dentists about how much mercury actually was outgassed from my fillings.
Even if a filling or onlay would be over 50% of your tooth , you can still avoid a crown.