How to show an associated value from a selected record in Rails 4.0 form

I am fairly new to RoR, currently working on a little free app for amateur sports clubs managers that help them to keep track of payments. This app shows relevant player data and a form to collect fees (tarifa) payments. I want to show the fee for the concept being paid before the payment is entered. This is the app view with the form and the HTML generated: form for pagos

this is the pagos_controller.rb 'new' action:

def new
  @pago = Pago.new
  @conceptos = Concepto.all
  @jugador = Jugador.find_by(id: params[:jugador_id])
end

and this is the views/pagos/_form.html.erb rendered into the views/pagos/new.html.erb :

<%= form_for(@pago) do |f| %>
 <% if @pago.errors.any? %>
  <p> All the error showing logic is omitted here for simplicity </p>
 <div class="field-tm">
   <%= f.label :concepto %>
   <%= f.collection_select :concepto, @conceptos, :name, :name %>
 </div>
 <div class="field-tm">
   <%= f.label :tarifa %> <div id="la-tarifa"> </div>
 </div>
 <div class="field-tm">
   <%= f.text_field :cantidad, :placeholder => "Cantidad a pagar" %>
 </div>
 <%= f.hidden_field :jugador_id, :value => params[:jugador_id] %>
 <div class="actions">
  <%= f.submit "Registrar Pago", class: "btn btn-default"%>
 </div>
<% end %>

This is the pagos schema...

create_table "pagos", force: :cascade do |t|
  t.string   "concepto"
  t.decimal  "cantidad"
  t.integer  "jugador_id"
  t.datetime "created_at", null: false
  t.datetime "updated_at", null: false
end

And the Conceptos schema:

create_table "conceptos", force: :cascade do |t|
  t.string   "name"
  t.decimal  "tarifa"
  t.datetime "created_at", null: false
  t.datetime "updated_at", null: false
end

I need to get the value selected in the 'collection_select' to use it to find that concept's tarifa like in

@conceptos.find_by(name: theConceptoName).tarifa

and put it in the

$("#la-tarifa").text(tarifa)

to show the value, but don't know how to get a 'theConceptoName' variable that works from the

<%= f.collection_select :concepto, @conceptos, :name, :name %>

I've tried several ways and I came up with jQuery way to get the value, so it would be something like this:

$("#pago_concepto").change( function(){
  var miConcepto = $("#pago_concepto").value();
  do the @conceptos search for miConcepto and get the tarifa;(*)
  $("#la-tarifa").text(tarifa);
});

but then how do I do the query(*) within the pagos.cofee code to get the tarifa value from @conceptos... I have done a lot of google research but every answer I find confuses me more. Please help me.