What is the cheapest way of gaining Inherent bonuses?

Question

A staple of high-level characters in D&D is boosting their abilities with inherent bonuses; up to +5 for each ability.

There are essential two methods of obtaining those bonuses:

1. The Manuals and Tomes; a Manual of Bodily Health +5 costs 137,500 gp.
2. Wish spells cast in immediate succession; 5 Wish spells in immediate succession cost 25k XP to the caster¹.

There are various dubious methods which can be found on the Internet, such as using distilled joy or liquid pain to offset the XP cost or conjuring Effreti and milking them off their Wishes. Such methods generally rely on a push-over of a Dungeon Master².

Baring a push-over of a Dungeon Master, what is the cheapest way of gaining Inherent bonuses?

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¹ Hiring a spellcaster to cast Wish costs 26,530 gp = 17 (CL) * 90 gp (9th-level) + 5 gp/XP * 5,000 XP, for a total of 132,650 gp for +5; though as per the rules of goods and services even a metropolis is not guaranteed to have a spellcaster able to cast 9th-level spells. Also, it seems that a spellcaster with 25k leftover XP, and thus a 25+ level spellcaster dabling with Epic spells, is unlikely to be moved much by a mere offer of 130k gp.

² Where push-over is defined as giving an advantage to PCs that its own NPCs are denied. For example, if the PCs use Effreti to get free Wishes, then there should be no reason that affluent NPCs would not also use Effreti to get free Wishes. Or if Liquid Pain is on the table, then clearly Evil NPCs would use Liquid Pain to achieve their objectives, ... As a result, answers should attempt to limit DM's intervention as much as possible, knowing that anything the DM allows for the PC may be allowed for their foes going forward (if the DM doesn't outright refuse).

Answer 1

The cheapest way I know of involves using the Twin Spell (+4) and Repeat Spell (+3) metamagic feats to gain 4 times the effect of a single Wish spell for the cost of 1.

It clocks in at 25,955 gp for +4 Inherent Bonus to a single ability or 50,955 gp for +5 Inherent Bonus. Respectively costing 24% and 38% of regular Wish usage.

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Unfortunately, applying metamagic feats on 9th-level spells is complicated, so metamagic reducers or cooperation is necessary:

• An Incantatrix 2 can apply both metamagic feats on the fly, while the spellcaster casts the spell.
• An Incantatrix 2 can apply one metamagic feat on the fly, while the spellcaster casts the spell with a metamagic rod to apply the other.
• ...

Thus, the party Wizard 17 could cast Wish using a Greater Rod of Repeat Spell, expending a reasonable 5k XP, and hire an Incantatrix 2 to apply Twin Spell for them:

• A Greater Rod of Repeat Spell costs 121,500 gp, which is easily amortized across the whole party.
• The most straightforward entry for an Incantatrix 2 is Wizard 5/Incantatrix 2, or a 7th level spellcaster. A 4th-level spell would cost 280 gp, which seems a reasonable price since Cooperative Metamagic does not expand any resource.
• The difficulty is succeeding on a Spellcraft check DC \$66 = 18 + 3 \times \left(9 + 4 + 3\right)\$ which requires minimum expenditure 675 gp¹.

All in all, assuming conservatively that each of the 4 party members raise only 2 abilities each, and therefore the rod is used 8 times before being sold again, each ability costs:

• Wizard XP: 5k.
• Part of Rod cost: 7,594 gp (Repeat) or 10,065 gp (Twin).
• Incantatrix cooperation: 280 gp.
• Scrolls: 675 gp.

This gives us the following costs:

     +-----------+------------+------------+
     | No Rod    | Repeat Rod |  Twin Rod  |
+----+-----------+------------+------------+
| +4 | 25,955 gp |  33,549 gp |  36,580 gp |
+----+-----------+------------+------------+
| +5 | 50,955 gp |  58,549 gp |  61,580 gp |
+----+-----------+------------+------------+

¹In detail:

• Guidance of the Avatar (Cleric 2, CL 3, Spellbook Archive): +20 Competence. Cost of scroll: 150 gp.
• Divine Insight (Cleric 2, CL 10, Spell Compendium, p. 70): +15 Insight. Cost of scroll: 500 gp.
• Skillful Moment (Sor/Wiz 1, CL 1, Dragon #350, p. 78): take 20 on roll. Cost of scroll: 25 gp.