Several times a year, I buy two or three 1kg bags of supermarket value table salt, and at the moment, it would cost you 25p/kg in either Tesco or Sainsbury's. I use it very simply to keep my drains running clear, and the method is a breeze.
(image from mysupermarket.co.uk)
First, snip off a corner of the bag and carefully pour enough salt in to the plug hole so that it stands there like a little mountain of salt. It will begin to slump into the holes, so I keep pouring until it stands still. Then I go and fill the kettle with as much water as it will take. The salt does seem to need to sit there for a few minutes, so don't try to be efficient and boil the kettle first! When the kettle is boiling, grab it and pour it onto the heap of salt. It will bubble a bit, glug a bit sometimes, but your drain will run clear within minutes. If it is really bunged up, I have done this a couple of times, maybe using half the bag of salt altogether, but I have never failed to clear a drain with this method.
Even if your drains aren't blocked, I like to do this once a month to keep them fresh. It costs pennies, and is much better than using a bottle of something stinking and chemical-based. [OK, I have a daughter doing A level chemistry and I know salt has a chemical name, but you know what I mean ;)] It is heaps cheaper too!