If you don't want to modify the dataframe, you could use a custom formatter for that column.
import pandas as pd
pd.options.display.float_format = '${:,.2f}'.format
df = pd.DataFrame([123.4567, 234.5678, 345.6789, 456.7890],
index=['foo','bar','baz','quux'],
columns=['cost'])
print df.to_string(formatters={'cost':'${:,.2f}'.format})
yields
cost
foo $123.46
bar $234.57
baz $345.68
quux $456.79