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Your second attack from a higher BAB will always be the main BAB -5. So in your case it is 7/2.

First you must pay for a +1 enchant on a MWK weapon. Which would be 2000gp. After that you can spend 6000GP to add the corrosive enchant, making it a +1 corrosive weapon. Which all told makes this an effect +2 cost wise.
For the Endless Ammunition enchant, you would need to pay the cost to bump a +2 to a plus +4. Which means you would have a +1 corrosive endless ammunition weapon.