>>> num=['276.30','1,446.90','23,456.80']
>>> "%.2f" % sum(map(lambda s:float(s.replace(',','')),num))
'25180.00'

>>> '{:,.2f}'.format(sum(map(lambda s:float(s.replace(',','')),num)))
'25,180.00'

http://stackoverflow.com/ques...

>>> def sum(*args):
...     r = 0.0
...     for n in args:
...             r += float(n)
...     return "%.2f" % r
...
>>> sum('1.1', '2.2')
'3.30'
>>> sum('1.1', '2.2', 3.3)
'6.60'

要看你需要的是显示出来的保留两位小数，还是真实的保留两位小数相加。
前者是

ans_print_version = "{.2f}".format(a + b + c)

后者是

ans = round(a + b + c,2)