My yard is eroding away and taking my house with it: what's the cheapest way to build a retaining wall to stop it?
I've recently purchased used railroad ties from home depot for my own landscaping needs.  They were the cheapest by volume that I could find about, $15 for an 8"x10"x8' (nominal), and if you want something that's going to match the worn look of an existing railroad tie retaining wall, well that's as good as it gets.

Concrete blocks will be more expensive and probably will stand out.  If you're not going to cement and rebar them in, I also find blocks harder to get set in so they look professional.