No it's not cheaper than Li6. As far as the heavy and fissile transuranics go, it is the cheapest stuff around though, and you're going to need something heavy like DU or possibly Beryllium for a tamper layer if you want your Li6 secondary to cook off. DU is not only a byproduct of the enrichment process to produce the fission primary device (so it's already on-hand when the weapon is near completion) but in the process of reflecting/containing the neutron flux it also undergoes fast fission because the neutron flux is so damn high at the point when the secondary is fusing. More bang, less buck. That's why DU is used for tampers. Tungsten and Beryllium work well enough, but they don't give you a nice 40-45% yield bonus from fast fission like DU will, and they are also more costly.