You will need to override account invoice class method that is responsible for sub price computation, the method name is  _compute_price(self):

you can d o this in by inheriting the sale.order.line

@api.depends('product_uom_qty', 'discount', 'price_unit', 'tax_id')
def _compute_amount(self):
res = super(InheritSaleOrderLine, self)._compute_amount()
"""
Compute the amounts of the SO line.
"""
for line in self:
line.price_subtotal = (Here you can give your own value)

return res