There is a great deal of consensus re: armor mods, but not so much when it comes to their order of importance. I add mods to my armor in this order: 100 lower reagent cost (obviously), 40 lower mana cost (lmc), faster cast/faster cast recovery (2/6 for mage or necro, 4/6 for spellweaver), 10 to 12 mana regen, resists in the 60s.
For PvM, you can throw together a suit pretty easily. My PvM suits usually include the following arties: kasa of the ra-jin (head), pendant of the magi (neck), rune beetle carapace (chest), stitcher's mittens (hands), ornament of the magician (bracelet), quiver of infinity, totem of the void (tally), cloak of death, arcane boots (to invis w/o lrc, if self-rezing), crimson cincture, scrapper's (book), chaos shield museum of vesper replica. I then add custom arms, legs, and ring that will bring me to 100 lrc, 40 lmc, 4/6 (or 2/6 without weaving), and around 10 mr. You definitely want AT LEAST 100 lrc, 40 lmc, and 2/6. You can probably get away with less mana regen, as wraith form will keep your mana pretty full. These items aren't cheap, but are readily available. You can probably put this suit together for around 30 mil. Of course, if you go with more custom pieces and spend more time shopping around, you can probably do a similar suit for 15.
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If you move into the PvP realm, a necro/mage suit can be quite expensive. Typically includes crystalline ring (16 mil on atlantic), ornament of the magician (10 mil), armor of fortune (12 mil), folded steel reading glasses (10 mil), inquisitor's resolution (30 mil), one-handed mage weapon w/ dci and no casting penalty (20+ mil), and custom pieces. This is a pricey suit, indeed.