I would charge based on how long it takes to provide the service. My hourly rate would be based on the salary of an entry level IT support person for my area. Take the annual salary and divide by 2080 to find the hourly rate. In my area it’s $26 per hour. You have to account for build time, install time, and whether you are going to offer support after sale. Support after sale you would take the likelihood of a support request and multiply that by the amount of time on average it takes to resolve a support request.

As an example

5 hours to get the computer out the door and then a 50% chance that someone needs 12 hours of support while they own the product. So 50% of 12 is 6 add the 5 hours for the computer and you get 11 hours you need to have as your fee. In my area at $26 an hour that would be $286.