There are info products on Clickbank about "how to make money doing surveys". I would send them an e-mail with an affiliate link to a product like that.
Just a quick calculation: let's say open rate will be 40% and 1% will buy the product where you get $20 commission... that get's you potential of earning $48,000!
Secondly, I would advertise on "solo ads" services and describe your e-mail base as people who like surveys (and stuff related to surveys - whatever it may me).
Third - send them e-mail to some nice article or whatever about surveys once a while - once per 2 weeks at least, so they won't forget you and unsubscribe in case you e-mail them after few months when they'll forget who you are and unsubscribe.
That's my 2 cents

Good luck!