If I'm paying for dreampress, do I also have to pay for a shared site for development subdomains?
Yes, you would need a separate shared hosting plan and pay for both that and DreamPress. It would be as if you had an account that hosts sites on both shared and VPS, you pay for both plans.
Is this a reason to keep it on godaddy? Or will you have some pricing model in the future to 'encourge' me to keep everything on DH?
Well... you'd be paying for Us and GoDaddy then.
Do you need your dev subdomains to be on a real server though? I tend to do my dev work on MAMP (so I can play with them on planes)