Motherboard design layout, prototype manufacture (initial run of 15pcb's) and hardware licenceing will probably cost you about £10k, probably about the same for a GPU (using existing GPU silicon (AMD/Nvidia), big problem as GPUs are bga not socketed. You then face the problem of getting the first production batch manufactured, you'll have to find a great deal of the production/parts cost either upfront or on a proforma basis ( you pay as soon as the product is complete not on a 1 or 2 month credit invoice so you either have to have advance sales or a large bankroll. And that's just the start, there is also case design, test equipment and design, compatibility testing, soak testing, ess testing, emf testing and certification, ce certification, product warranty and liability insurance and a whole load of other things to consider. I've worked in the electronics industry for 30 years so I know what's involved in getting a working product to market and it's not quick and definitely not cheap or easy.