Change items costs & stats

To better reflect value differences between Armour, Helm & Shield. 
As well as promote different Armour setups

Armour Worth most in terms of cost & effectiveness
Shield worth second in terms of cost & effectiveness
Helm worth least in terms of cost & effectiveness

Max defense = 24% with a full set of gold. 
Which should be balanced as there are now 3 items to purchase instead of one
Merge pull request #355 from SlappyBag/master

Item cost & stats change for better balance and variance