Thin Controller Fat Model (and calculations)

I'm trying to elegantly add some functionality to my app. It's a
language school that has an enquiry form. the functionality I am trying
to build is pricing. Basically I'd like to calculate a price for a
student and store it in the enquiry. Prices come in two flavours, Course
Price (course price per week * duration), Accommodation Price (accom
price per week * duration). There are one off booking fees for both.

So, how do I do all this maths in my Model (I read everywhere that the
model is the place to do it) and keep my controller clean.

Here's the story so far.

Parameters passed when the form is submitted...

Parameters: {"commit"=>"Create",
"enquiry"=>{"address1"=>"g",
"country_living_in"=>"Germany",
"date_of_birth(2i)"=>"1",
"accomodation_duration"=>"4",
"address2"=>"jkg",
"date_of_birth(3i)"=>"1",
"title"=>"Miss",
"address3"=>"g",
"postcode"=>"gj",
"course_duration"=>"2",
"body"=>"sdfds",
"english_level"=>"Advanced",
"accomodation_id"=>"1",
"address4"=>"gj",
"country"=>"Germany",
"firstname"=>"sdf",
"requested_start_date(1i)"=>"2009",
"mobile"=>"gjk",
"requested_start_date(2i)"=>"2",
"phone"=>"jk",
"requested_start_date(3i)"=>"3",
"surname"=>"sdfdsfbjk",
"course_id"=>"2",
"email"=>"gjk@dsf.com",
"nationality"=>"Germany",
"date_of_birth(1i)"=>"1950"},
"action"=>"create",
"controller"=>"welcome"}

My Enquiry Fields (the final price should eventually make it into
total_fee).

?> Enquiry.column_names
=> ["id", "created_at", "updated_at", "title", "firstname", "surname",
"responded", "body", "photo", "phone", "mobile", "email", "address1",
"address2", "address3", "address4", "postcode", "country",
"requested_start_date", "date_of_birth", "nationality",
"country_living_in", "english_level", "accomodation_id",
"accomodation_duration", "course_id", "course_duration",
"course_booking_fee", "course_fee", "accommodation_booking_fee",
"accommodation_fee", "total_fee"]

I think I need something like this, am I on the right lines?

My Proposed Enquiry model

I think that this callback way is a good one.
But it's better to make the validations first, no? You can use some
callback that do the things after validations. For example before_save
or before_create.

Here for more: http://apidock.com/rails/ActiveRecord/Callbacks