Create a real currency field with the benefits of filtering due the nature of the field.
Make sure your system meets the minimum requirements:
The currency fieldtype works just like any other fieldtype, it also looks like a normal fieldtype but in fact it is not a real simple field. The currency fieldtype convert in the MySQL database to a decimal field. This way it can hold any currency value like 9.95 or 188.50
Because the field is now converted to an decimal, the filtering (orderby) in the {exp:channel:entries}
works perfectly
With the decimal settings you can convert the point to a comma.
Because the currency fieldtype convert the field in mysql to a decimal field, it is wise to delete this field if you want to change it to an other fieldtype. Otherwise that field is an decimal field and not a text field.