Great question; if you're hiring a marketing agency to run your email campaign, you can typically expect to pay $200 to $500 per month.

This would include services like email design, copy, backend management, and segementation.

While this may seem pricey, it's recommended that you spend 15% to 20% of your marketing budget on your email campaign; this is because it's a highly cost-effective strategy that, when run correctly, can provide a $30-$45 to $1 ROI.

However, there are some cheaper alternatives to manage your email campaign using software like Mailchimp or Constant Contact that provide design templates and easy backend control but require ongoing management if you're looking to have a weekly or monthly newsletter.

Alternatively, hiring a freelancer to provide some of the services previously mentioned might be more cost-effective for a small to a medium-sized enterprise looking to test the waters. 

I hope this answers your question. :)