I think a lot depends on what type of site is being advertised. If it`s of a non-commercial variety like a blog then I don`t really see much point in spending money promoting it.
Adwords can run away with your money so set a realistic budget and test the waters first. If the traffic isn`t there, or if the sales don`t merit using paid advertising then time to reconsider a few things, one of which is whether your site is set up right in the first place to make visitors want to stay and buy.
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