Cheapest TT shop online?

It really depends on which brand. If chinese brands, eacheng. Net is good. If stiga in asia that would be pingpongonline. Com

As Yogi_Bear said, it does indeed depend on brand. Butterfly for example (like Apple !) are very hot on what I will call "resale price maintenance" although it is strictly speaking illegal if it's called that - in other words they don't allow anyone to sell their gear below a certain price. In the UK I've always found Bribar's prices to be the best for many brands, and Thorntons are also worth looking at particularly if you're fond of Stiga. If you like Chinese equipment there are bargains to be had at Table Tennis Hub, so long as you have the patience to wait for stuff to be shipped from Thailand or China - I just bought a Li Ning shirt, that took about a fortnight to arrive.

If you're looking into web sites outside the UK then you'll really have to do your own research - however a lot of people seem to swear by Tabletennis11, but you'll need to watch the Euro exchang rates to properly understand the prices.

Which country are you from?