‘Brown’s Gas Book 2’ and it’s Resources are all you really need < $50. I do suggest getting other support material but it is optional.https://www.eagle-research.com/cms/store/browns-gas Cost to build (from the plans in Brown' Gas Book 2) is about $500 plus $1 for each liter per hour desired. This varies somewhat depending on your skills and resources. The 'Brown's Gas Book 2' and associated Resources book tells you what need to design a BG electrolyzer to fit your needs and where to get everything you need to self-build a machine. Your self-build machine (from my plans) will out perform ALL commercial machines currently on the market.