The basic formula I use is materials cost plus labour plus overheads plus profit = wholesale price x 2 = retail
materials = kit and blank and box
labour = how much you value your time at x time taken
overheads = workshop costs + consumables (abrasives, finishes etc.) + marketing (labels etc.)
Profit = up to you, I work on 5%
So if the slimline set costs include labour and profit that would make your retail price £29.56 based on my system which is about what I would charge if selling privately, through the gallery I would add on another 20% for their fees making it £35.47.