To get down to say 750kg, you don't get something for nothing. Short of building every bit of the whole ship out of carbon/kevlar (which would make it cost £200k+) .
The all carbon aircraft cost about 50K all in and come in at about 230 270kg. Mine is 253kg and passes CS-VLA (ie it is not going to fall apart). This is the situation as of today, not some theoretical guess of what might be.
From an LAA / BMAA perspective we have many more strips than at any time, we have much more capable aircraft, much lower fuel and maintenance costs. For the fun flyer life has never been so good!
Rod1