Passing Info to a form with a URL

So basically, what I want to do is to be able to construct URLs to send clients and when the click them to go to the form, whatever I have added into the URL will populate the form fields.

So for instance, I know I am charging them $15/month for hosting, I want to be able to construct a URL that will put 15 into the form field for monthly cost.

From my research on here I would have thought that

.com/form-page/?cost_month=15

would have worked. (I did checkmark "Allow field to be populated dynamically" and set the field to be cost_month)

It doesn't work for me though. What am I missing?

I did read this page too http://www.gravityhelp.com/documentation/page/Allow_field_to_be_populated_dynamically

Many thanks for the help

http://limecuda.com/hosting-agreement/

Here is what the URL field looks like: http://screencast.com/t/bTrl02XX

Not sure if this complicates things but this is the nightly build of WP. It is using postname as the url http://screencast.com/t/VPndn9p7wA - i resaved it in case that was it.

It was 1.5.3 but I went ahead and updated to the 1.6 RC5

I would do something like http://limecuda.com/hosting-agreement/?site_url=http://clientsite.com&?cost_month=25

So it would populate with their url and the price i have given them.

That failed too :(
http://limecuda.com/?page_id=755/?cost_month=15

I wouldn't be able to live with that either of course.

I can give you access to this site or I can just live with it.
Seems quite odd.

With help from Chris I figured this out. Turns out there was a little setting checked in Yoast's SEO plugin that said:
"Redirect ugly URL's to clean permalinks. (Not recommended in many cases!)"

This was one of those cases. So if anyone has a problem with that query being stripped out, make sure you don't have that setting checked if using that plugin. Dumb mistake on my part having that checked.

Thanks Chris for all your help!