The cheapest method is to purchase the Excel spreadsheet from the IEEE. The cost will be around £400 unless you are a member of the organisation. Arcaware has utilised the spreadsheets but be aware that it is a very time consuming method and very easy to make mistakes. You will require an accurate fault level analysis and operating graphs for protection relays and fuses.
Next up in the price range are packages like ArcPro. These will carryout an arc flash analysis as well as providing details on arc flash boundaries and PPE ratings. They are slightly more user friendly than using a spreadsheet but will still require a high level of concentration.
Arcaware has not utilise this software so cannot report on it in depth.
The major downside to these methods is that an inaccurate fault level study will produce an inaccurate arc analysis. Generally fault level studies give a higher fault level than there actually is, this gives a lower arc energy rating from the software.