Ruby on Rails Update Price When User Selects Value from Drop Down

I have a cinema system in Ruby on Rails and am currently working on the booking functionality. When a user makes a booking they can choose the number of adult, child, senior, and student seats from a drop down menu:

<%= f.select :adult_seats, '0'..(@booking.showing.screen.remaining_seats > 10 ? 10 : @booking.showing.screen.remaining_seats).to_s, id: 'Seats' %>
<%= f.select :child_seats, '0'..(@booking.showing.screen.remaining_seats > 10 ? 10 : @booking.showing.screen.remaining_seats).to_s, id: 'Seats' %>
<%= f.select :senior_seats, '0'..(@booking.showing.screen.remaining_seats > 10 ? 10 : @booking.showing.screen.remaining_seats).to_s, id: 'Seats' %>
<%= f.select :student_seats, '0'..(@booking.showing.screen.remaining_seats > 10 ? 10 : @booking.showing.screen.remaining_seats).to_s, id: 'Seats' %>

What I want to be able to do is have a text box at the bottom of the page that will update the total price based on the user's select. So if they select 2 adult seats the text would say 20.00, the difficult part is that the prices are in a separate table, so I need a method of retrieving the prices and then updating the text.

This is my full views/bookings/_form.html.erb:

<%= stylesheet_link_tag    "application", media: "all", "data-turbolinks-track" => true %>
<div id="contentWrapper">
<div id="content">
    <div class='large_panel'>
        <table>
        <form id="prices">
            <%= form_for @booking do |f| %>
                <%= f.hidden_field :user_id %>
                <%= f.hidden_field :showing_id %>
                <%#= hide_action @booking.showing.screen.remaining_seats %>
                <%= image_tag @booking.showing.film.image_url,:size => "900x250" %>
                <h1>NEW BOOKING:</h1>
                <tr>
                    <td width="280px"></td>
                    <td width="100px">
                        <br><%= f.label :adult_seats, 'Adults:' %>
                    </td>
                    <td width="100px">
                        <br><%= f.label :child_seats, 'Children:' %>
                    </td>
                    <td width="100px">
                        <br><%= f.label :senior_seats, 'Seniors:' %>
                    </td>
                    <td width="100px">
                        <br><%= f.label :student_seats, 'Students:' %>
                    </td>
                    <td width="100px">

                    </td>
                </tr>
                <tr>
                    <td>
                        How many seats do you require:
                    </td>
                    <td>
                    <input type="hidden" value="#{@booking.showing.screen.remaining_seats}" name="seats_available" id="seats_available">
                        <%= f.select :adult_seats, '0'..(@booking.showing.screen.remaining_seats > 10 ? 10 : @booking.showing.screen.remaining_seats).to_s, id: 'Seats' %><br>
                    </td>
                    <td>
                        <%= f.select :child_seats, '0'..(@booking.showing.screen.remaining_seats > 10 ? 10 : @booking.showing.screen.remaining_seats).to_s, id: 'Seats' %><br>
                    </td>
                    <td>
                        <%= f.select :senior_seats, '0'..(@booking.showing.screen.remaining_seats > 10 ? 10 : @booking.showing.screen.remaining_seats).to_s, id: 'Seats' %><br>
                    </td>
                    <td>
                        <%= f.select :student_seats, '0'..(@booking.showing.screen.remaining_seats > 10 ? 10 : @booking.showing.screen.remaining_seats).to_s, id: 'Seats' %><br>
                    </td>
                    <td>
                        <div class="actions">
                          <%= f.submit 'Book Showing' %>
                        </div>
                    </td>
                </tr>
            <% end %>
        </form>
        </table><br><%= @booking.showing.screen.screens_info %> <%= @booking.film_age_rating %>
    </div><%= render "/error_messages", :message_header => "Cannot save: ", :target => @booking %> 
</div>
</div>

And the schema:

create_table "prices", force: :cascade do |t|
  t.text     "name"
  t.decimal  "price"
  t.datetime "created_at", null: false
  t.datetime "updated_at", null: false
end

 create_table "bookings", force: :cascade do |t|
  t.integer  "user_id"
  t.integer  "showing_id"
  t.datetime "created_at",     null: false
  t.datetime "updated_at",     null: false
  t.integer  "adult_seats"
  t.integer  "child_seats"
  t.integer  "senior_seats"
  t.integer  "student_seats"
  t.integer  "disabled_seats"
  t.integer  "immortal_seats"
end

Can anyone help.