Trello list scraper that extracts all numbers from boards and logs the sum
It is good to see that you are using the if __name__=='__main__': guard. However, I think you are doing a bit too much in your main and I would make a get_total_month function like this:

...
def get_total_month(month, board_lists):
    costs = 0
    month = month.lower()
    for entry in board_lists:
        if month in entry.name.lower():
            for card in entry.cards:
                costs += float(card.name.split('-')[1])
    return costs


def main():
    board = conn.get_board('BE89pW61')
    total = get_total_month('august', board.lists)
    print 'total is {}'.format(total)


This way it will be easier to incorporate for example something like:

totals_per month = [get_total_month(month, boards.lists) for month in months]


which is then easy to plot e.g. with matplotlib:

import matplotlib.pyplot as plt
...

totals = [get_total_month(month, boards.lists) for month in months]
months = range(len(months))

plt.plot(months, totals)
plt.show()


Adding the actual names of the month (and maybe also their year) is a bit more involved, but you can have a look at the examples here and here.

Regarding whitespace, try to stick to one convention, preferentially the one set by pythons offical style-guie, PEP8. This means in particular not writing sum (total) or float( entry ), but sum(total) and float(entry).