So far it looks as if the best price I can come up with for mailing one post card to my entire list is about $475, which comes out to about $.47 a post card.

For the most part you have to send out hundreds of letters/post cards to even get a bite though.

Also for the record, I personally prefer post cards.

Yellow Letters, Mail, Postcards?

I'm strictly using postcards now but should probably start using some online methods at some point. 

Next month I will send mail to the same 700 people; though I might send postcards for the second round.

I just don't want to send these postcards and put out bandit signs and do all the research and lose money I don't have to unnecessarily.

Either call them up (you can find a lot of numbers on www.WhitePages.com, but not every number), or send them a direct mail sequence offering something of value, just to get them to contact you.I never door knocked, but it would be better than sending a letter or postcard or package.Also, search expired listings from 3 months ago, 6 months ago, a year, etc. 

It's the only logical explanation for questions like...What service is available to mail out postcards for me?  

Yellow letters, professional letters, zip letters, text postcards, pictorial/full color postcards, greeting cards...etc, will all appeal to different people.