You should use number_with_precision helper. See doc.
Example:
number_with_precision(1.5, :precision => 2)
=> 1.50
Within you form helper:
<%= f.text_field :cost, :class => 'cost', :value => (number_with_precision(f.object.cost, :precision => 2) || 0) %>
BTW, if you really want to display some price, use number_to_currency, same page for doc (In a form context, I’d keep number_with_precision, you don’t want to mess up with money symbols)