you need to understand concept of fields and what that field contains:
- field float contains a float number, usable imediatley
- many2many contains ids (from relation table) , you need to get the desired value of other model first...
so you could for example:

for m2m in many2many_field:
    res[m2m.id] = field.float * m2m.some_value_field
    # NOTE: some_value_field  need to be of type float or integer , othervise you will get an error

wich wil return a list of m2m field values multiplied with field.float...

may the source be with you!

You got to understand the symentics of each data type and its behaviour...

So from your above code, I suppose you are trying to add all taxes ... But Tax_id is of type M2M, which is nothing but again a list of browse records.. hence you got to loop throug it to perform your action...

Your code should be like this....

for record in self.browse(cr, uid, ids,context):
     for tax in record.tax_id:
          res[record.id] += record.price_subtotal * tax.amount   

Do refer, standard functionality/ calculations  say Invoice, Purchase... etc so you will get a better idea of it...