About safety there is not much to say - China southern is as safe as any other big international carrier - and safety in general does not have any direct relation to price.
In fact , skytrax has a rating of 4 stars for that carrier ( including safety ), and it's quite a good one.
There are different safety reports and rankings from different organizations that I can quote here - and you can easily look them up on the net - though some argue you shouldn't.
As a personal experience - After using CZ for more than 50 different flights I can testify that it does not have "cramped planes" any more than any other airline and in fact they have a relatively new fleet at least on International and premium national destinations ( Between major airports ) - but then again, every airline has better and worse equipment and your specific one flight will not determine the quality of the fleet.
..and Food is a matter of taste ( I am personally not a fan ) but also that changes by destination, origin , season and time of flight .
Regarding the price - there could be multiple reasons like special regional promotions for your country, government regulations in the case of china, seasonal promotions or just date availability .. none has to do with safety.
P.S> Small Remark as a rule of thumb : if an airline is a member of a major alliance ( for example Skyteam which is the case for CZ or Star alliance ) you can assume they are "better" then those who are not - and that is simply because they are required to comply with that alliance standards on top of all "regular" standards. That includes safety, service, amenities etc..
of course this rule of thumb also has exceptions - I had great flights with non-alliance carriers and bad ones with major alliance members - but as a general rule it works ..