Getting value of a text Feild for ajax request Rails -

i have simple rails form looks

 <%= form_for :phrase, url: phrases_path |f| %>        <p>         <%= f.label :"enter text" %><br>         <%= f.text_area :text %>       </p>         <%= f.fields_for :order |builder| %>       <p>         <%= builder.label :"ваш email" %><br />         <%= builder.text_field :email %>       </p>   <% end %> 

this form goes appropriate controller , in controller depending on value(text) of text area performs calculations create price , save order.

and works fine. have requirement. before submitting application user can click on button(inserted somewhere between form) know cost. said above cost calculated using text of text_area , logic present inside some method. implement button. on clicking button ajax request started, value(text) of text area retrieved , cost calculated depending on some method. calculated value shown on page.

i know start ajax request , update page. dont know how value of text_area ajax request. cannot apply in javascript since computation of cost requires some method ruby/rails dependent