code help

Hi
I have the following Javascript adapted from a previous post by Jane. I only have 2 fields - 'design' and 'cost'

<script>

{literal}

var x = document.editform.value_design;

x.onchange = x.onkeyup = function() {

var design = document.editform.value_design.value;

var cost = document.editform.value_cost.value;

cost = parseFloat(design) * 2;

document.editform.value_cost.value = cost;

} {/literal} </script>

This works well with the cost field being updated to 2design. However there is one problem - I want the 'cost' field to be read only. If I do this the normal way in the properties the javascript will not work. So I tried the following from a previous post.

<script>

{literal}

var x = document.editform.value_design;

x.onchange = x.onkeyup = function() {

var design = document.editform.value_design.value;

var cost = document.editform.value_cost.value;

cost = parseFloat(design) 2;

document.editform.value_cost.value = cost;
} {/literal} </script>

However this caused 2 problems...

1. The 'cost' field is only disabled after an update to 'design' so the user could type manually
   
2. On save the 'cost' value reverts back to the previous value - it is not updated to the new value even thought the field show the new correct value throught the Javascript.
   So I tried the following...
   

   <script>
   

   
   {literal}
   
   [color=#FF0000]document.forms.editform.value_cost.disabled=true;
   
   var x = document.editform.value_design;
   
   x.onchange = x.onkeyup = function() {
   
   var design = document.editform.value_design.value;
   
   var cost = document.editform.value_cost.value;
   
   cost = parseFloat(design) * 2;
   
   document.editform.value_cost.value = cost;
   } {/literal} </script>

This sorted out (1) above but the problem (2) still exists - the 'cost' field always reverts back to the old value.
Can anyone help as this is driving me mad - my first experience of Javascript and so close!!!
Thanks Gareth