I made a battlemage-ish character, and I don't want to invest in magic any more than I have to as I rely more on my mace and shield than I do my summons, but I want expert and master summons as well. What is a good number to shoot for? How about reducing the cast cost?

EDIT: I'll be more specific, what is a good number for total magicka to shoot for?

0 magicka get enchanting and reduce conjuration costs even with 100 magicka I was able to cast expert spells with 50% reduction but I advice you get the twin souls perk two is better than one and you can get it for 4-5 perk points

Master summons will of course benefit from the master perks. All of my Necro/Summoners shoot for a naked Magicka of 200-250 pending enchantments and other spell casting.

Do I need to perk enchanting to get 100% cost reduction? That seems a little game breaking - how about 50% reduction?

You can get 50% conjuration cost reduction without enchanting easy. Necromancer's amulet is 25%, Vokun is 20%, Master Robes are 22%, Archmage robes are 15%.

Best master conjuration spell is Dead Thrall and is 593 magicka at skill level 100 with no magicka cost reduction gear or master perk.

As I said though, my character is a battlemage. I am wearing heavy armor. No robes.

conjour perks reduce cost by 50%, dead thrall with master conjour perk is about 250 i think, so probley 300 magicka would be fine.

You don't cast Dead Thrall in combat though so that isn't an issue.

I think you need 108 magicka to summon a single Dremora Lord with the Expert perk, 100 Conjuration and no cost reduction gear.

At 100 conjuration you need 550-650 mana to cast master level spells (each one varies a little). Adjust that for whatever gear you plan to wear and if you plan to take the mana reduction perk. Keep in mind that the master spells are permanent though so you can switch out gear to cast them without it getting too anoying to do so.