Using an online 3D printing service is the cheapest way to 3D print, as you don’t have to invest in expensive 3D printing equipment, you just have to order your 3D printed parts.
Some 3D printing techniques are more affordable and accessible than others. FDM is one of the most afforable and accessible technique, as machines and 3D printing materials are cheaper. Naturally, industrial 3D printing technologies such as Multi Jet Fusion (MJF) or Selective Laser Sintering (SLS) requering bigger and more expensive 3D printing machines. However your choice will depend on the nature of your project, your manufacturing requirements and the result you are excepting.