I think that the problem consist in a discordance between the name of the parameters.
In your jquery code you use:
location.href = "Products/helloPrice?ProductId=" + productId;
(Please note that the parameter is
ProductId, with both the initial P and the I in uppercase)
but in your controller the parameter is defined:
public ActionResult helloPrice(int productid)
(Please note that all the name is in lowercase)
I think that the solution is just to call the two parameters exactly the same (including upper o lowercas) in both the controller and the jquery code, that is:
a) ProductId in both
b) or productid in both
I hope this will help you.