19" Rota's are about 0.5-1.0kg heavier than OEM standard 18's. Weights are taken with tyres, half worn on OEM, new on Rota's, so the Rota's (on they're own) are probably a bit lighter considering the extra 3mm of tread on the tyre and the extra width of the rubber.
Rota's are just styling wheels, nothing lightweight about them. If you want 19" lightweight you're talking near £2k plus rubber. Ray's are by far and away the cheapest way to get into lightweight rims, but that's not what you're looking, you're after OEM looking 19's, something i've never looked for because they are fugly compared to GTR's, Grids etc etc