3000 per day from other channels is really healthy.
You could easily spend 5-10K on retargeting alone on display banners, but you should/could also do native ads (traffic to your blogs/content) and then also pre-roll video and later, connected tv or radio.
If you want to do more than retargeting you could easily spend 20-30K more on prospecting depending what your Facebook/Google mix is currently.
What category? Ecom or Leads?
If you have brick and mortar you can also track store visits as a KPI.
The problem is GDN is not the best for your overall mix and scale for retargeting. It can be OK for prospecting (still way too expensive in my opinion) but it’s fairly weak for retargeting compared other solutions.
Have you looked at any other display platforms? Talked to any agencies about advanced programmatic/rtb? Someone to segment creative by funnel stage.
The new hotness is multi touch attribution that ties it together with your search and social for deduplication.
How is your in house creative capability? HTML5 tend to perform best.