There was a similar question in our user group in which I responded:
We made a wireframe of a master template and asked an external designer to make a visual design in Sketchup (or Photoshop). We send the sketchup file together with an instruction document to https://www.emailmonks.com/ (NO AFFILIATION). With some communications to discuss some minor adjustments we received an HTML file to be used as a master template in Marketo.
Costs: 1000 EURO for the Designer and 350 EURO for the HTML work.
But you will need to understand to Marketo Email 2.0 to implement as you will need to cut the master into different templates.The HTML work is executed in offshore so be clear in your instructions and ask for a conference call to explain (if needed). If you use this approach it is advised to understand how Email Editor 2.0 works with modules and variables.
Our original master template consists of 30+ modules which eventually cut in to multiple predefined templates.
Hope this helps.