I'm not sure I understand the contract terms. Is a 1-month soul plan one where she takes your soul after 1 month? Or is it that you loan your soul out to her for 1 month, and during that month get the power? If the former, those plans are terrible. I've hardly ever been in a situation where I could talk to a bat, so losing my soul after just a month just so I could get that power would be garbage.
But if it's the latter, being able to get powers in exchange for lending my soul for only a period of time seems quite tempting, actually.
I assume it isn't permanent, but being able to see in the dark for the low-low price of lending my soul out for 3 months seems like a bargain. And I assume I can not renew the contract if I ever start to value my soul more than the ability to see in the dark.