Controller method using JS doesn't return/render data when called within modal popup

I've been stuck with this problem a couple of days, and I didn't find anything that could help me.

The idea is to get price_scenarios index view rendered in a modal popup. For each price_scenario shown in the table, a controller method, "price" is called. This method gets some info from the model and render it formated using JSON. This way of getting the prices was coded for a previous developer who is no longer in the project, and is necessary becouse price values can take a while to be calculated, so a small wheel gif is rendered when page loads.

The problem: price_scenarios "price" method doesn't render any value. This only happens if "price" is called within the index view rendered in a modal popup, I've tried doing this rendering price_scenarios index and it works fine. Also, this method is succesfully used in many other views of the app. I think it must be a problem with format responses, due to call the method from a popup, but everything I've tried haven't improved the results.

product show.html.haml a button displaying price_scenarios index as a modal popup:

 %h3
   = model_class.human_attribute_name(:price)
   = link_to PriceScenario.model_name.human(count: 2), formula_product_price_scenarios_path(@formula, @product), remote: true, data: {toggle: "modal", target: "#price_scenarios"}, class: "btn"

 #price_scenarios(class="modal hide fade" role="dialog" aria-labelledby="Delta" aria-hidden="true")

price_scenarios_controller.rb definition of index and price methods

class PriceScenariosController < ApplicationController
  before_filter :require_client
  load_and_authorize_resource :formula
  load_and_authorize_resource :product, :through => :formula
  before_filter :get_tuple, only: :price

  def index
    @price_scenarios = @product.price_scenarios
    respond_to :js
  end

  def price
    @price_scenario = PriceScenario.find(params[:id])
    @price = @price_scenario.shifted_price(@tuple)
    @price_units = @product.price_units

    respond_to do |format|
      format.html { render partial: 'price'}
      format.json { render json: {price: @price, units: @price_units}}
    end
  end

price_scenario index.js.haml view renders index view in the modal:

$("#price_scenarios").html("#{escape_javascript(render partial: 'modal_price_scenarios')}");

_modal_price_scenarios.html.haml view rendered in the modal, "price" is called within it.

#modal_price_scenarios(class="modal" role="dialog" aria-labelledby="Delta" aria-hidden="true")

  - model_class = PriceScenario

  .modal-body
    %table.table.table-striped
      %thead
        %tr
          %th= model_class.model_name.human
          %th= model_class.human_attribute_name(:price)
          %th= model_class.human_attribute_name(:references)
      %tbody
        - @price_scenarios.each do |scenario|
          %tr
            %td= scenario.name
            %td.price{:"data-product-price" => price_formula_product_price_scenario_path(@formula, @product, scenario)}
              = image_tag('ajax-loader.gif')

-Price.html.haml partial rendered by price_scenarios price method.

= "#{@price} #{@price_units}"

product_price.js

$(function(){
  $('[data-product-price]').each(function(){
    var url = $(this).data('product-price')
    $.ajax({
      url:      url,
      cache:    false,
      dataType: "json"
    }).done(function( data ) {
        var priceWithUnits = data['price'] + ' ' + data['units']
        $('[data-product-price="' + url + '"]').html(priceWithUnits);
      });
  });
});