Lets say you have a staff with a 1st level spell, a 2nd level spell, and a 4th level spell (all minimum caster level and none of which require expensive components). The price of said staff would be 13,200 gp (400 gp * 4 spell level * 7 caster level + 300 gp * 2 spell level * 3 caster level + 200 gp * 1 spell level * 1 caster level) and cost 6,600 gp (13,200 gp/2) to craft.
Your numbers are wrong. A staff (or any other magic item) only has 1 caster level. Each spell going into a staff doesn't get to have a different caster level. Also, staffs have a minimum caster level of 8 - you can't have a staff with a caster level below that.
So your example staff would be:
4th level spell: 400 x 4 x 8 = 12,800
2nd level spell: 300 x 2 x 8 = 4,800
1st level spell: 200 x 1 x 8 = 1,600
Total of 19,200 gp.
Adding a 6th level spell to it (Which raises the minimum caster level for all spells to 11th):
6th level spell: 400 x 6 x 11 = 26,400
4th level spell: 300 x 4 x 11 = 13,200
2nd level spell: 200 x 2 x 11 = 4,400
1st level spell: 200 x 1 x 11 = 2,200
Total of 46,200gp
PS. How much would a cantrip/orison cost to be crafted into a staff? Would it be crafted as a one half level spell with a 1st level caster?
Cantrips are priced as if they had a spell level of 0.5. Actual cost to add a cantrip would vary based on the staffs caster level, as well as how many other spells are in the staff.
PPS. Is a staff cast cantrip/orison treated as a prepared cantrip (at will) per charge used? or is it a single casting? how many charges does it use?
It would use charges the same as any other spell. Minimum of 1 charge per use, like all other spells in a staff.