Force from MIC (not to be confused w/ Hank's Energy Bow) is a +2 market price. So its cost depends on what your bow is currently. If it's a +1 bow, then you're paying the difference between that and a +3 bow to upgrade it, for instance. Price to enhance magic weapons is (bonus^2) * 2000 gp, or half of that for the creation cost (creating also costs xp, of course).
So a +1 enhancement is 2000 gp market price. A +3 is 18000 gp market price. Difference of 16000, and you pay half of that in gp and 1/25 of that in xp.
If you're magically enhancing the weapon from scratch (for example, a +1 Force Composite Greatbow), then you'd be paying 1/2 of the 18000 gp in gp costs and 1/25 of it in xp costs. The bow itself would cost...whatever the hell a greatbow w/ the relevant strength modifier costs... plus 300 gp for masterwork. Plus 300 gp if you wanted Elvencraft. And so forth.
For your edit 1: You have to give the weapon at least a +1 before applying Force to it.
For your edit 2: When used as a melee weapon, it'd be treated as quarterstaff, so no proficiency = -4 penalty. It'd still be treated as a bow when firing arrows.