Yeah you're doing it wrong. You take rares from 4-5 close stations, go 160+ LY away to another area where there are 4-5 close stations sell there, buy rares there, go back to your starting point. That's the idea in a Cobra.
For T-6 you do some grand circuit of 15-71 (some routes around here in the forum, 71 station route is more sightseeing with bonus profit really) stations or so, buying rares as you go and selling at the furthest station away. The idea is to always have your hold always at least half full of rares.
Both have profitabilty around 1M Cr / hr, an Asp can be higher at 1.5M Cr / hr (if you're lucky).