It's way more complicated to calculate how to sell a good at a certain price than if you sold your existing system on eBay so I will try to explain it this way:
The easiest method for this is to first calculate your cost of goods manufactured. To do that it's:
Cost of direct materials (as in what you purchased)
+ cost of direct labor (hours you spend building PC x hourly rate)
+ overhead (equipment setup and testing, shipping, etc)
+ Beginning inventory (work in process)
- Ending inventory (completed goods)
-------------------------------------------------
Cost of goods manufactured
So if your direct materials are £751.03, then it would be calculated this way:
£751.03 (direct materials)
+ £50.00 (direct labor, estimated)
+ £75 (overhead, estimated)
+ £1000 (beginning inventory, estimated)
- £750 (completed goods)
--------------------------------------
£1126.03 is the base price that can be determined for the base sales price.
If you want to calculate exactly there's lots of other factors to take into account like machine hours to determine overhead rate (total budgeted overhead / basis), what your beginning and ending direct materials and labor are and so on and so forth.