I'm not much of an expert, but maybe this can be useful: AlN is a somewhat widely used insulating substrate that is chosen where sapphire is insufficient (~40 W/mK), but BeO (~300 W/mK) is too expensive or toxic. The intrinsic conductivity of single-crystal AlN is very high (~320 W/mK), but the material is extremely difficult to grow into large single crystals, so sintered substrates are used instead. This reduces thermal conductivity to 170-230 W/mK depending on grade. Can't comment on pricing though.