Magic Staff Pricings

[jaker2003]

Ok, so I was trying to figure out how to price magical staffs and found page 287 of the DMG. There it explained that the magic ability with
the highest Spell Level was priced at: (Spell level x CL x 750).
The next highest level ability is priced at: ((Spell level x CL x 750)*0.75).
Any remaining abilities are priced at: ((Spell level x CL x 750)*0.5).
Also listed was a note that an ability's price can be halved by increasing their charge usage to 2.

So I broke down some magic staffs to see if I understood the pricing system, but my numbers didn't match the listed price. [sblock=Some examples]

[b]Staff of Swarming Insects[/b]
Spell Level    Charges used    Spell Name                 [u]24,750[/u]
     2                1        [i]Summon Swarm[/i]               10,125
     5                3        [i]Insect Plague[/i]              11,250
CL:9th                                                    [b]3,375[/b]

[b]Staff of Transmutation[/b]
Spell Level    Charges used    Spell Name                 [u]65,000[/u]
     1                1        [i]Expeditious Retreat[/i]            4,875
     2                1        [i]Alter Self[/i]                          9,750
     3                1        [i]Blink[/i]                               14,625
     4                2        [i]Polymorph[/i]                         9,750
     5                2        [i]Baleful Polymorph[/i]              18,281
     6                3        [i]Disintegrate[/i]                     19,500
CL:13th                                                            [b]-11,781[/b]

[b]Staff of Fire[/b]
Spell Level    Charges used    Spell Name                 [u]17,750[/u]
     1                1         [i]Burning Hands[/i]                  3,000
     3                1         [i]Fireball[/i]                          13,500
     4                2         [i]Wall of Fire[/i]                    12,000
CL:8th                                                             [b]-10,750[/b]

The underlined number is the listed price.
The bold number (across from Caster Level) is the difference between the listed price and the sum of the staff's abilitys' prices. None of the spells used by the example staffs had extra costs from materials or XP, I double-checked the spell descriptions.[/sblock]

I'm looking for:
1) How does an ability's price scale with increased charge costs per use, such as a spell that uses 3 or 4 charges to cast from the staff?

2) Why can't I get the numbers to resolve?

3) What's the price on the Retributive Strike ability on the Staff of Power?