Okay mate, you're not an old China hand.

I bought tomatoes a few days ago. The price was 3 RMB/KG

Ask your wife about wholesale markets. You can buy there at lower prices than supermarkets, especially if you buy in bulk.

Can't do that in Blighty.

But in Blighty shopkeepers can use their cash and carry cards to buy all kinds of products at prices at around 30% of supermarket prices.

Another thing I'd suggest is that you don't mention your illegal teaching arrangements on this board in the future, for your own sake.
There are both blabbermouths and supersnouts, as well as Chinese government information gatherers, all of whom cannot be trusted, and could conceivably drop you in it.

Thanks for the advice again.