Which is most effective low Cost marketing technique ?

Training your staff  will avoide you cost in money and time 

Social media, massive emailing, public relations and phone prospection. 

Guerrila Marketing! If you have never heard this term before, I strongly advice you to google and try to understand it. 

The most effective low cost strategy is Direct marketing.

the best tehnique in my opinion is Cost benefit ratio

Pamper your exixting customer, digital marketing and public relations