Have a read in the NA section of this site for actual fuel econ figures used by members (not manufacturers claims). Personally if fuel economy is a purchasing decision in a car i would advise to not consider the skyline at all. Dont forget parts will cost more, generally labour costs more aswel, hardly any local support from manufacturer etc.
Also, if your magna changes fuel econ by 700ml per hundred (9.8 to 10.5), thats only about $1 at todays petrol prices. Remember thats 91octane. Lets say an R34 has the same increase for comparison sake (as you're trying to see how much more a skyline costs), well the difference would be about $1.20. Who actually gives a sh!t for $1.20 when they get to drive a skyline instead of a magna? (not being rude, just trying to help out).
On the flip side, are you getting that fuel consumption figure from the onboard computer? If so and its anything like my commodore (chances are that it is), that figure is actually false. The figure is based on when the battery was last out of the car or when the ECU/gauge was last reset. Its an average, so to make it go up by 700ml may mean the actual econ for that tank may be something like 18per/100 but the computer settles on 10.5 as it hasnt been reset in a year or two and is averaging the last 2 years of all driving. Know what i mean?
Sure you tried google? I just googled "r34+gtt+spec" and on the first page of results got this;
Model: R34 Nissan Skyline GTT
Model Year: 1998.5 to present...
Engine Name: RB25DET
Engine Cooling: Water cooled,
Engine Configuration: Inline 6 Cylinder,
Engine Camshafts: Double Overhead Cam 24 valve,
Engine Aspiration: Intercooled Turbo,
Engine Valve Technology: NEO Variable Valve Cam Timing Technology
Engine Bore × Stroke: 86mm × 71.7mm
Engine Displacement: 2498 cc
Engine Compression Ratio: 9:1
Engine Max Power 206kW (276 bhp) @ 6400rpm
Engine Max Torque 343Nm (253 ft-lb) @ 3200rpm
Fuel tank capacity: 65 litres
Fuel type: Unleaded premium (RON 98+)
Fuel consumption: 9.4 litres per 100 km
Transmission: Type 5 speed manual
Drive layout FR: Limited slip differential (as standard)
Steering system: HICAS 4-Wheel Steering
Steering type: Power Assisted Rack & Pinion
Front suspension: Independent suspension multiple link system
Rear suspension: Independent suspension multiple link system
Front brakes: Ventilated disk
Rear brakes: Ventilated disk
Front tyres: 225/45ZR17
Rear tyres: 225/45ZR17
Turning circle: 10.2 m
Dimensions: Exterior length 4580mm
Exterior width 1725mm
Exterior height 1340mm
Interior length 1780mm
Interior width 1400mm
Interior height 1105mm
Wheelbase 2665mm
Front track 1480mm
Rear track 1470mm
Ground clearance 140mm
Weight: 1410 kg
I highlighted the figures you're after to compare. Remember thats the turbo specs, so the fuel econ should be even better again
Using your figures and theirs for fuel econ only;
100k's in a magna costs you approx: $12
100k's in an r34 would cost you approx: $14
....depending on the slight variance in prices from area to area. The difference is due to needing higher octane petrol than the stuff your magna runs on
Moral of the story for people in the future: Do a search