You should be aware that there is a new kind of item called a Runestaff, found in the Magic Item Compendium. It is only for spellcasters. It allows you to cast certain spells a certain number of times a day (usually three but sometimes less), at your own caster level, by using a spell slot of an equivelent level to cast the spell. So for example if you had a runestaff that had the magic missle spell, you could swap out your already prepared sleep spell (or a spell of a higher level, like scorching ray) to cast magic missle, and you could do this swap up to three times a day.
There is a pretty solid pricing formula behind runestaves, and I could post if for you if you would like to see it. I think the kind of item you are talking about would work better if made as a runestaff, and would be a lot less expensive that what you are proposing.
Off the top of my head, I think it was the highest level spell squared times 400, plus the second highest level spell squared times 200, plus the third highest level spell times 200, etc...and then add it all up. So if you had a fire-themed runestaff of Fire Shield (4th level spell), Fireball (3rd level) and Scorching Ray (2nd level), the price would be 16 x 400 = 6400 + 9 x 200 = 1800 + 4 x 200 = 800 = 9000 total (I think I did that right), and each spell could be swapped for a spell of an equal or greater level three times a day. If you reduce the number of times a day, you can subtract 100 from the mutiplier for each reduction in the number of times a day (minimum 1) for the highest level spell (so it would be x 300 in this case for 2 times a day, or x 200 for 1 time a day), and 50 from the multiplier for the other spells (so x 150 in the case of fireball and scorching ray for 2 times a day, or x 100 for 1 time a day). And the runestaff spells must be ones on your spell list by the way, and I believe there are some other restrictions.
So, a runestaff containing the spells you mentioned (tweaked a bit to match 1 x day) would work as follows:
Create Greater Undead 1/day (Clr 8, Wiz/Sorc 8) 12,800
Control Undead 1/day (Wiz/Sorc 7) 4,900
Create Undead 1/day (Clr 6. Wiz/Sorc 6) 3,600
Animate Dead 1/day (Clr 3, Wz/Sorc 4) 900 or 1600 depending on who contributed spell
Death Knell 1/day (Clr 2) 400
Total: 22,600 or 23,300 gp, depending on whether you use a cleric or wizard to add the Animate Dead componant (I think i caclulated that correctly). However, the spell has to be on your spell list, and while almost all of these spells are on both spell lists, not all of them are (problem with Control Undead and Death Knell). Not that big a deal, particularly if you are a cleric with Control Undead on your Domain list. You could share the runestaff between a cleric and a Wiz/Sorc, though I believe it has some sort of attunement requirement.
Of course, you cast at your own caster level, and not 15th. And more importantly you need to use equivelent or better spell slots to cast these spells. So if you don't have an 8th level spell slot, you will not be using the Create Greater Undead spell until you do. But, at least this is a more objective price, and an affordable one, and one that grows in power as your character does.
