Generally all percentage reductions are additive to each other, and apply after the flat reductions. Sorcerer already has pretty cheap spells, so cost reduction, either flat or percentage base are not that beneficial. On the other hand Breton is very good on classes & roles that need to spam relatively expensive spells, for example a Templar healer.

Dark Magic: Unholy Knowledge gives 5% cost reduction
Light Armor: Evocation gives 2% cost reduction per piece, so a minimum of 10% since you'll run at least 5 pieces of light.
Breton: Magicka Mastery will add another 3% cost reduction.

Force Pulse base cost is 2700 magicka, 2295 for the Altmer sorcerer, 2214 for the Breton socerer. That's 81 cost reduction per cast. But things should take into account your base recovery and all skills you typically cast into a rotation, and also if you get support, from healer with orbs.

Since Sorcerer deals most damage as elemental, predominantly lightning, the 4% damage done from Altmer: Elemental Talent will boost your damage a bit. It stacks with other percentage increases, such as Storm Calling: Enegized 5%, and CP: Elemental Expert, let's say 20% with 56 points spent there. So the former boosts your elemental damage by ~3.2% on an Altmer compared to a Breton. The damage penalty will be less on classes that deal a bit more damage as magic damage, such as Templar or NB. Mind that even those do deal a large chunk of their damage as elemental damage since they use Elemental Blockade and, for the most part Force Pulse as spammable (starting patch 4.x even NBs).

Overall I'd say Breton is better for PvP and as healer, Altmer is better for PvE damage dealer.