So I think those are my two favorite skills aside from two hand, and I want to use them both with one character, and am thinking about this for a build:
http://skyrimcalculator.com/#136288

I already have this character going at level seven investing in archery, light armor and sneak so far.

My concern is that I won't have enough magicka for higher level conjuration spells. What amount of magicka should I aim for if I want to be able to cast the master thrall spells?

What about if I stick with expert spells?

BTW, this does no enchanting.

Bump.

The base cost for the master conjuration spells.. I'll list base magicka for all thralls,im guessing you'll just use necromancy spell but just incase.
Dead Thrall is 1000 base , or 500 with master perk
Flame Thrall is 900 base, 450 with perk
Frost Thrall is 1100 base, 550 with perk
Storm is 1200 base, 600 with perk

In short if you are going to use the master conjures you will need to use perk or the cost will be too high, although you don't need to cast often as the thralls do stay with you until death.
If you compare that with the Expert level spells...
Dread Zombie 302 base, 151 with expert level perk
Storm Atronach 322 base, 161 with perk
Dremora Lord 358 base, 179 with perk
however these only last 60seconds, and people reanimated with Dread Zombie will turn to ash after that time

-edit- Also take into consideration that magicka costs are reduced when you increase your corresponding skill.. for example 100 conjuration will mean the cost of casting is 60% of base so, Dead Thrall is 1000 base, but at 100 conjuration it'll be 600, with perk at 100conjuration it should be 300.

As a necromancer, the only master spell you need is dead thrall, and that only needs be cast once, and not in combat. If you are worried about magicka for that, you can activate a shrine of Julianos, put on any fortify magicka items temporarily, and use fortify magicka potions. remember that 1000 points is base cost, not cost with 100 skill (not sure, don't trust me, I think it's 592).

Thanks so much - that was very helpful.

So I would be fine sinking maybe seven points into magicka for the expert level spells, but I wonder if I will be able to put enough in for the master ones? Seems like it takes a LOT. What do you think?

With 100 skill in conjuration, which reduces cost by 41%, and the perk, which reduces that by 50%, you won't need much.
Take the Atronach as your stone, put a few points in, and you should be fine.

As was said 300 magick as a master of conjuration with the relevent perk can cast the dead thrall spell.
Enchanted items can help raise your magick as well as potions and the atronach stone.
I would put about 10-15 points into magick just have some extra for the expert spells.
From there you can work on alchemy for fortify magick potions or take the atronach stone.

My vampire character is like that. Hes my favourite!