Sounds about right to me. I bought my 54 plate (Dec 04) 1200GS in December with 13k, ABS, spokes, heated grips, handguards, fender extender and cylinder guards for £5295.
Mine's in great condition for its year and I was well pleased with what I paid.
Remember, even bikes of this age are selling for close to £7k in BMW dealer forecourts at the moment so anything under £6k is good value for money I say, especially with a low mileage.
If the bike's in great condition, I'd be prepared to pay that bit more as it shows it's been looked after.