Assuming cheap means not $$$ but time. Here's the best ROI.

1. Content Marketing: Blogging & Video - The reason this ones the best is that it has a long lasting effect (SEO). You can build up a following of potential customers, and even if you pivot your product, the customers should stay the same - so you can re-direct them to the new product. It always continues to drive traffic after you stop blogging. If you through in some guest post on top of that - it can be quite effective (ex: KissMetrics blog, BufferApp blog, Hubspot, etc for great examples). Marian knows best http://clarity.fm/marianschembari

2. Press / Media. This is the second best but it's very much a hits thing (sometime it drives qualified traffic, some times it doesn't). The key is to start with smaller outlets, then work your way up to getting covered by NYT, CBC, etc. Ryan Holiday is the master at this: https://clarity.fm/ryanholiday?b=h

P.S. Those are the only 2 in my mind that are "cheap" although they do cost time (which is money $$ in my book). However, even better then both of those is a relentless focus on building a great product or service that people can't help but talk about. That's the cheapest in my opinion.