I started small, let things pay for themselves before buying more. My initial investment was about $10,000, and that was just for a handful of GPUs, some parts like motherboards and CPUs, an electrician for the 240V 30A line, PDU, etc. Once I made that money back I slowly bought more. My most recent purchase has been the AvalonMiner which will pay for itself in another six months or so. If I include the revenue from all the mining hardware though, the AvalonMiner will be paid for much sooner. It’s funny how that works :)