Allow only positive decimal numbers

You could do something like this :

# .....
class priceForm(ModelForm):
    price = forms.DecimalField(required=False, max_digits=6, min_value=0)

This, also, is responsible for the validator value of 'price'.

In Django 2.2 you can add constraints to a model which will be applied in the migrations as a constraint on the database table:

from decimal   import Decimal
from django.db import models

class Item(models.Model):
    price = models.DecimalField( _(u'Price'), decimal_places=2, max_digits=12 )

    class Meta:
        constraints = [
            models.CheckConstraint(check=models.Q(price__gt=Decimal('0')), name='price_gt_0'),
        ]

Note:

Validation of Constraints

In general constraints are not checked during full_clean(), and do not raise ValidationErrors. Rather you’ll get a database integrity error on save().

Use the MinValueValidator.

price = models.DecimalField(_(u'Price'), decimal_places=2, max_digits=12, validators=[MinValueValidator(Decimal('0.01'))])