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:

1. You are confusing roi with payback period.
2. There are intangible factors to consider like command of your data, control over security, etc.
3. Opex and capex are values used for fixed-term projects, they aren’t meant to quantify ongoing efforts past project outset.
4. 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.
5. 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.