I think you have to apply to be a Technology Partner - Premier Partner Program - Technology Partners - Marketo
Ooooh you just want a sandbox account. You can reach out to your Account Manager and they can hook you up. You might still have to pay though, but not nearly as much.
I would raise a support case under the My Stuff > My Cases area or talk to your Marketo rep.
I do not recall if Sandboxes come free though....
Sandboxes do come at a cost but its not much....you can integrate your Sandbox with your CRM sandbox or production whatever you wish.
There is no developer edition for Marketo however Home » Marketo Developers helps you to understand the development features in Marketo and can help you in many custom integrations.
Sandboxes aren't free, but I wouldn't pay more than 8% of your database contract cost for one. Also, upon renewal, make sure they aren't counting your sandbox records as part of your total records because they've tried to do that to us before, and then tell us we were over our database size limit.