This is neat, and I see that lots of folks are getting a good look at their laid out setup costs. However, you should stop using project management budget coding words. Also:
- You are confusing roi with payback period.
- There are intangible factors to consider like command of your data, control over security, etc.
- Opex and capex are values used for fixed-term projects, they aren't meant to quantify ongoing efforts past project outset.
- Hardware is absolutely NOT a "one-time purchase". If you are amortizing the cost of equipment, you must include projected failure rates, particularly for disks. Please refer to green-cycling.
- Information projects are different than physical processing projects because they "expire", meaning your i5-7200 is OK now, but will likely be nearly useless in 2030 when everyone is pretty much moved to 4k.