tahat is what I say magnetic separation requires lots of power, but quality is ideal. no mater how contaminated plutonium is you will have perfect separation in one pass. and since only tiny amount is required electricyty cost for few bombs will be not too high, but of course much higger than military equvivalents.
having perfect plutonium with better grade than military, you can make quite good bomb. as I know if you use neutron reflector you need only 3kg of plutonium instead of 30kg uranium, so the bomb will be tiny too. as I know smalest military designs are about 50kg or little less. with 0.5-2 kiloton power.

magnetic separator does not require much knowledge to build, and produces perfect results as I know at least 99.9% purity. so it is best choice for small scale production.