Actually the thrust is lost. Reverse isn't so much a function of exhaust and fan gasses being directed forward (many reverse systems don't direct gasses forward at all), but rather a cessation or interruption of thrust. Gasses moving "aft," which provide for forward thrust of the aircraft, are interrupted or re-directed by vanes and doors, to preclude their pushing the airplane along.
At the same time, the engine is spooled to a higher value.
The net thrust produced by the engine is the exhaust value, minus any drag or friction from the engine. This applies to a turbine engine as much as any powerplant.
Drag produced at the engine inlet during the reverse process doesn't go away, simply because the engine is no longer producing thrust. Take away the rearward thrust, and all you have left is the drag.
Exhaust and bypass gasses being directed forward or sideward contributes a small measure to the slowing motion of the aircraft, but not a substantial amount. The retarding action of reverse thrust is a function of inlet or intake drag, rather than re-directed gasses.
Airplane thrust reversers (Henry Spencer; Mary Shafer)
I can't speak to the A380, but on the B747, we use an autobrake system to calculate landing distances, which has three settings. Each of these settings produces an acceleration (I'm not a physicist, so I still like to think in terms of deceleration) rate of a particular value. That is to say, on the "Minimum Autobrake" setting, the aircraft decelerates at 4 feet/second/second. At "Medium Autobrake," the airplane decelerates at 6 feet/second/second. This value is calculated by the antiskid system, regardless of aircraft weight. At the "Maximum Autobrake" setting, full braking pressure is applied, stopping just short of a wheel skid.
Brake energy is a big issue; stopping a big, heavy airplane results in hot brakes. Large, hot brakes take a long time to cool,which affects the turn-around times (the time between landing and departing again). Airplanes get paid to fly, not to sit and cool, so brake energy is a big issue. An airplane can't take off again until the brakes are cool enough, both for issues with hot brakes after takeoff, and for the possibility of a rejected takeoff.
The FAA doesn't permit the use of reverse thrust in calculating landing performance. Therefore, when we perform TOLD (takeoff and landing data) calculations, reverse thrust isn't considered. We use it, however.
The value of reverse thrust will vary with temperature, engine condition, and the velocity of the airplane. Reverse thrust is more effective during the landing at higher speeds, than slower speeds. The velocity of air entering the engine is higher at higher landing speeds than slower speeds, and thus the inlet drag or overall drag effect is higher at higher speeds.
With the autobrake system, acceleration remains constant, but the amount of work done by the brakes is reduced as reverse thrust is used. In other words, at "Minimum Autobrakes," we're still slowing at the rate of 4 feet per second per second, but the amount of brake energy absorbed, and the temperature increase in the brakes, is substantially less with reverse thrust in use. The autobrake system doesn't care what's slowing the airplane, just that the airplane is slowing at the prescribed rate.
Consequently, we use reverse thrust as much as possible.
We're limited to 70% fan speed on the particular engines I operate for normal reversing operations. In an emergency, we'll use all the reverse we can, of course. We also reduce reverse thrust as the airplane slows down, in part because reverse isn't as effective, and in part because reverse can cause the engine to begin ingesting it's own exhaust gasses at slow speeds, or cause debris to be blown up from the runway, where it could be ingested into the engine.
Landing with 70% fan speed for reverse is roughly equivalent to minimum autobrakes, requiring very little braking pressure by the autobrake system. The value it contributes is somewhat subjective, and I can't give you a hard number; it varies with field conditions. The part it actually plays in the landing roll also varies with aircraft weight; a heavy airplane may require more braking action than a light one, where reverse thrust stays relatively constant.
Sorry, I can't give you any specific values.