Based on the recent experimental results of the LHD and the magnet technology–cost basis developed for the ITER construction, the design windows of helical reactors are analysed. For searching design windows and investigating their economical potential, we have developed a mass–cost estimating model linked with the system design code (HeliCos). We found that the LHD-type helical reactor has the technically and economically attractive design windows, where the major radius is increased as large as for the sufficient blanket space, but the magnetic stored energy is decreased to a reasonable level because of lower magnetic field with the convenient physics basis of the H factor near 1.1 to the ISS04 scaling and beta value of 5%.