To replace commas with periods in text for decimal numbers in Python, you can use the `replace()` method of the `string` class.

Here is an example of how you could do this:

text = "The total cost is $1,234.56"

# Replace commas with periods
text = text.replace(',', '.')

print(text)

This will print the following output:

The total cost is $1.234.56

Keep in mind that this will only work if the commas are used as decimal separators. If the commas are used as thousands separators, you will need to use a different approach to correctly convert the numbers to decimal format.