how to caculate total price automaticatically based on selected product in nintex forms

where do you store unit price? in an other list?

then use lookup()  function to query for unit price by the model selected and multiply the result with given quantity.

If you have a UnitCost field in another list called Products, and a list lookup control in your form called Product List, and field named Quantity, the formula in the calculated field for total price would look like this:

lookup('Products', 'Title', parseLookup(ProductLookup), 'UnitCost') * Quantity

In this case the select lookup value is matched against the Title field in the Products list. If you need multiple line items for the order it can get more complicated.

I am using cascading dropdowns like Product Line one DropDown,if we select that Product line in dropdown related product models are displayed in another dropdown for each and every product model unique price is there but we dont want to show price for each product model externally in form i have created Product line list as onelookup   and i create productmodel list in that i use lookup of productline, along with that product model column,

created another list called price list in that i create a productmodel which was a lookpup filed in that list and a price coloumn for each and every productmodel. i created a one transaction list  for store the details we provide in form wtih theese coloumns like Productline,productmodel,quatity,estimated price in form if i select productline value in dropdown related producmodels are displayed in productmodel dropdrown if select value in product model dropdown and  after provide quantiy value  manually in text filed in form i need to get estimated price automatically i have used  calulated value in from to get estimated price. i tried in diffrent process but not estimated value is not geeting how could i acheive it.

i am doing same only waht you mention but i am not getting i have created price and prodcutmodel in one list,price related to each productmodel which was a lookup column in that list 

I believe following formula should give you the price

lookup('PriceList','ModelLookupField',parseLookup(ModelCtrl,false),'ModelUnitPriceField') * QuantityCtrl‍

It might help to break down the problem and test each step individually. I've posted more information here http://www.tekdoginc.com/blog/updating-total-price-in-sharepoint