I\'ve currently got a large AI guided torpedo with a couple hundred blocks of rock as a kinetic warhead, a casing for the core compartment, with disintegrators surrounding the core in a sphere and a single thruster group behind it all. The most expensive part is the disintegrator warhead surrounding the core, which you can replace with whatever else you want, shields or even more rock to keep things on budget. With the only blocks costing anything being the computer guidance provided by the 1-block pulsator, the ship core, and it\'s thrusters, cost no longer becomes a major issue. It\'s a very cheap way of making sure the enemy ship now has a big gaping hole in it and a lot less power/shield blocks, maybe even a missing computer or two.
I find that these are not only a good tool to bypass traditional capital ship defenses, but can also be used to ward off unwanted engagements, as most pilots that see a torpedo coming after them will try to get rid of it as quickly as possible, most likely moving away from it to give themselves time, letting you get some distance.
Hope I helped explain some of the strengths of torpedoes.