The value added(and therefore cost), in thousands of gold pieces, to armour is equal to the total equivalent bonus multiplied by itself, plus silly extras that are just there to confuse people and make powergamers squeal with joy...
So a +3 shield with arrow deflection(equivalent to +2) and arrow catching(+1) would cost:
3 + 2 + 1 = +6 equivalent enchantment
6 x 6 x 1000 gp = 36000 gp in addition to the cost of the shield itself, which is usually insignificant by now.
So for 36000gp a shield can be enchanted to have a +3 enhancement bonus(+3 enhancement bonus to shield armour class and the shield is more resistant to sundering), Arrow deflection(You get a reflex save to avoid 1 arrow per turn), and arrow catching(Arrows aimed within 5 feet of you target you instead). The three properties do not actually influence each other, but they still work well together to make a character able to protect their friends and resist the consequences. Often people will add the required +1 enhancement bonus and then dedicate their efforts to gaining special properties. This is exacerbated by the presence of spells which add enhancement bonuses to equipment...
Weapon enchantments are twice as expensive as armour enchantments, so if the above shield were a weapon the upgrades would cost 72000gp instead.
But, long story short, each goody is separate, so the +2 has no effect on the flaming and the flaming has no effect on the +2.
If you want to read the version that people were actually paid to write, it can be found here:
http://www.d20srd.org/srd/magicItems/magicWeapons.htm