Passing value from input form

Rails 3.1.3

I would like to pass the value in an input form to a parameter in
function.

The value is to be set in

<%= text_field_tag :price %>

A PayPal official icon initiates the action, 'checkout'.

<%= link_to
image_tag("https://www.paypal.com/en_US/i/btn/btn_xpressCheckout.gif"),
:action => 'checkout' %>

Clicking the icon above should pass the value entered in the form.
I tried the following

def checkout
    contribution = params[:price]
    setup_response = EXPRESS_GATEWAY.setup_purchase(contribution,
       ...
    )
    redirect_to EXPRESS_GATEWAY.redirect_url_for(setup_response.token)
  end

And a callback function as well.

  def complete
    contribution = params[:price]
    purchase = EXPRESS_GATEWAY.purchase(contribution,
       ...
    end
  end

But the transaction gives an error saying the value is empty. I guess
that I am not passing the input value properly.

Could anyone show me how to set the local(?) variable that can be passed
to the function, 'checkout' as well as 'complete' ???

soichi