Sean K. Reynolds, formerly of WotC and now of Paizo, estimates one feat to be worth 25K.
Of all the things possible with the polymorph spell, spending it on an archer so as to boost his str has to be one of the worst uses ever...
Come on... Admit it!
Arrow Demon
Abilities: Str 21, Dex 19, Con 29, Int 14, Wis 16, Cha 12
Close Combat Shot (Ex): An arrow demon does not incur attacks of opportunity for firing a bow while threatened. In addition, the arrow demon can shoot its bows to make attacks of opportunity as if it threatened the squares adjacent to it with its bows.
Oversized Weapons (Ex): Arrow demons can use bows of any kind that are up to one size larger than their size would allow without penalty.
Symmetrical Archery (Ex): The arrow demon has an amazing ability to wield two bows at once in unison. Any time the arrow demon could make a single attack with a bow, it can attack with both of its bows, at -2 on all its attack rolls that round. For example, when making an attack action, an arrow demon can fire one arrow from each bow at its normal attack bonus minus 2, and when making a full-attack action, it can fire one arrow from each bow for each of its normal iterative attacks (two arrows from each bow, each shaft at a -2 penalty). The statistics block above already includes these bonuses. The arrow demon takes no additional penalties for using multiple weapons as long as it is using only bows.
Before making any hasty decisions, I suggest you ask the spike first whether it can live through the impact.
We have a bow that can survive being sundered by an Orc with a Greataxe. I think the spike will be ok.
As for your Hardness and HP remark, let me clarify: It is irrelevant. You are not asked to break the rocky surface underneath your feet, you are asked to shove a spike in it.
Great! Then that should make the task easier!
...But in case someone can truly pull this off... then... what can I say... He's probably wasting his talent being an archer.
Holy moving goalposts, Batman!
To the OP:
A rereading of Bone Bow leaves me wondering if it actually auto-adjusts to your strength modifier. It is possible that it does not, in which case you should look into the Bow of the Wintermoon, a Relic from the MIC, which can be found on Pg. 48. It costs 3400 gp and starts of with a +1 enhancement bonus. (You don't necessarily have to care about the actual relic bonuses). It does have an alignment restriction though (CG/NG/CN).
You can attempt to reverse-engineer the Wintermoon Bow in order to figure out the costs for the Str-adjusting property. Let's ignore the relic bonuses, to keep things simple (since they require further expenditure of resources to activate anyway).
1) 3 400 * 10/7 = 4 857 (Take out the alignment restriction discount)
2) 4 857 - 2000 = 2 857 (Take out the +1 enhancement bonus)
3) 2 857 - 400 = 2 457 (Remove the cost for a basic masterwork composite longbow.)
Now you can round up the value to 2 500 gp, or round it down to 2 450 gp.