The three spells to build the higher level spells upon would be:
20. Imbed X
25. Daily X
30. Constant (up to tenth level)
Say we make the relation between enchantement spells and their source spells linear from there on, that would mean an "Imbed N" spell would have twice the maximum level (so "imbed 13" would have level 26, and "Imbed 39" would have level 78). "Daily N" would then be 2.5 times the level of N (so a 13th level daily would require a 33rd level enchantment spell, and a 39th level daily would require a 98th level ebchantment spell), and a "Constant N" would require three times as high levels for the enchantment as the spell to be imbedded (so 39th level for a 13th level spell to be embedded, and 117th level for a 39th level to be embedded). Clean and simple.
Depending on how the distribution of levels is in your game world, that allows conclusions about just how rare such items must be. And of course, what the price of such items would be, as per TC.