Okay, I'll bite.

Bow for an archer:

L2 (+1/2,000gp): -

L4 (+2/8,000gp): +1 Pitspawned Composite Serren Greatbow
2,000 = +1
4,000 = Serren wood (BoED p38) gives its arrows Ghost Touch as an Ex ability
1,000 = Pitspawned template: +2 to confirm critical hits
7,000 TOTAL

L6 (+3/18,000gp): No upgrade.

L8 (+4/32,000gp): +1 Force Slow Burst Hideaway Pitspawned Composite Serren Greatbow
Force (+2): Makes fired arrows into force attacks. Ignore DR, Wind Wall, and so on. Does not affect creatures immune to force effects.
18,000 = +3
4,000 = Serren wood
1,000 = Pitspawned
5,000 = Slow burst: On crit, target slowed for 3 round.
2,000 = Hideaway: shrinks to two sizes smaller; easy to conceal.
30,000 TOTAL

L10 (+5/50,000): +1 Force Sizing Slow Burst Hideaway Pitspawned Composite Greatbow + Strongarm Bracers
30,000 = Total so far
5,000 = Sizing: Weapon changes size to fit you
6,000 = Strongarm Bracers: Wield a weapon one size larger than normal at no penalty.
41,000 TOTAL

L12 (+6/72,000): +1 Force Sizing Slow Burst Hideaway Pitspawned Composite Greatbow + Strongarm Bracers + Ring of the Darkhidden
41,000 = total so far
2,000 = Ring of the Darkhidden: Invisible to darkvision.
43,000 TOTAL

L14 (+7/98,000) +1 Splitting Force Sizing Slow Burst Hideaway Pitspawned Composite Greatbow + Strongarm Bracers + Ring of the Darkhidden
43,000 = Total so far
54,000 = +3 to +6: Splitting (+3): Each arrow fired splits into two as it leaves the bow. The attacks are rolled separately. Bonus damage applies in full to both arrows.
97,000 TOTAL

L16 (+8/128,000) +1 Distance Splitting Force Sizing Slow Burst Hideaway Pitspawned Composite Greatbow + Strongarm Bracers + Ring of the Darkhidden + Ring of Sustenance
97,000 = Total so far
26,000 = +6 to +7: Distance: Double range increment.
3,000 = Ring of Sustenance: Bearer is nourished without food or drink. 2 hours of sleep is as restful as 8.
126,000 TOTAL

L18 (+9/162,000) No upgrade

L20 (+10/200,000) +1 Collision Distance Splitting Force Sizing Slow Burst Hideaway Pitspawned Composite Greatbow + Strongarm Bracers + Ring of the Darkhidden + Ring of Sustenance + Bracers of Wind
126,000 Total so far
64,000 +7 to +9: Collision: +5 damage on any successful attack.
10,000 Bracers of Wind: -2 to any ranged attacks against you; +1 to your ranged attacks (competence bonus).
200,000 TOTAL

Final result (assuming a Medium character):
+2 attack
2d8+6+Str damage [Force]
20/x3 crit; +2 to confirm crits; slow for 3 rounds on crit.
240' range increment
Splitting: Effectively double attacks.

Ghost Touch as an Ex (non-magical) property.
Adjusts to your size automatically, should it change.
Shrinks on command to an easily-concealable size.
Invisible to darkvision.
-2 to any non-seige-engine ranged attacks against you.
Sustained w/o food or water; 2 hrs sleep/night.