[DahkCeles]

In that case, you might be able to get away with just using spellID 190356. I'm not sure if this corresponds to any particular ranks or classic vs retail... this is just grabbing it off wowpedia really fast.

The script below loads the spell data, and once the data is available it then creates a font string and scripts it to update any time your mana changes.

local spell = CreateSpellFromSpellID(190356)
spell:ContinueOnSpellLoad(function()
  local maxCost = GetSpellPowerCost(190356)[1].cost
  local f = CreateFrame("Frame")
  f:SetPoint("CENTER")
  f:SetSize(25,25)
  local fs = f:CreateFontString(nil, "ARTWORK")
  fs:SetAllPoints()
  f:RegisterUnitEvent("UNIT_POWER_UPDATE", "player")
  f:SetScript("OnEvent", function()
    if (manaCost) then
      fs:SetText(string.format("%d", UnitPower("player", 0)/maxCost))
    end
  end)
end)

Edit: disclaimer, I'm not a frost mage and I haven't tested this code.