Cheapest 1190 on Bike Trader is £7,000, in fact there are two at that price, both 2013, one with 23k and one with 30k = at 4 years old, assuming the dealer has about £1k on top of px price the trade in would have been £6k at 4 years old.
2014's are up at £9k in dealers so I would imagine a 2 year old one with 25k is likely to be worth at least £7k....
...but how much better would a BMW really be, the KTM comes fully spec'd up with all the Electronics, Cruise, Cornering ABS / TC, electronically adjustable semi-active suspension etc. I am guessing a GS of similar spec will be a little bit more expensive and not worth all that much more come px time....
...plus when you're looking at losing at least £3k a year regardless of the bike you choose and into account fuel / tyres / servicing etc the cost of owning either bike must be close to £5,000 a year to finance and maintain. Another @£400 - £800 of depreciation on the KTM is peanuts when looked at in the bigger scheme of things..
The 9k service interval of the KTM might lower the gap further (2 services in 25k Vs 4 services with the GS) my local KTM dealer is also a whopping £30 an hour cheaper of labour than my BMW dealer was 5 years ago.
If you want cheap do what I did last time, get a ex-demo / pre-reg Super Tenere, I paid £9400 for mine and traded it in for £6k four years later, that pisses all over the BMW's depreciation - and my 12 year old V-Strom is worth about the same as I paid for it 4 years ago, perhaps you want one of those?
So assuming you prefer the KTM you are looking at about £6 a week extra / 5% more to have the bike you want, I certainly would not pay 95% of the cost of the bike I want, to have a bike I do not want.