I think in general that weapon design, like ship design, should try to avoid pushing too many attributes too far up the line. The vanilla game options are the standard missile, the hardhead and the q-bomb (I'm not counting the energy bomb because I don't think it's part of this category of launched weapons, and it's never used against the player, either).
Missile: TL1, 30Cr
Hardhead: TL9, 350Cr
Q-bomb: TL6, 2500Cr (I'd be tempted to suggest TL12 and 5000Cr at least, myself, but that's a whole other argument)
I don't think the proposed torpedo is really in the same WMD league as the Q-bomb, so we're really just comparing it to the two missiles.
The Hardhead costs more than 10 times the standard rate, and is much harder to find. That said, it's vastly more useful, in that there's a good chance it'll hit what it's aimed at (or at least chase it over the horizon). Speed and accuracy are identical, as far as I know.
IIf the torpedo (or bolt, or Main Battle Ordnance, or whatever you want to call it) has a greater impact than a missile, and is also shielded from ECMs, that makes it substantially better than a Hardhead. All those pluses need a minus or two, at least. Slower, or less manoeuvrable, or both, would be good.
It's also worth thinking about how the player is going to encounter this, and what would be the most fun. Is it going to be launched at the player a lot? Then you'd want something that creates a few "ohcrapohcrapohcrap" moments as the player frantically tries to shoot it down before it hits. Make it too slow, or too inaccurate, and it becomes too easy to dodge: too fast, and too accurate, and it just becomes frustrating. Slower than a standard missile (maybe between 4.5 and 5.5?), but just as accurate and more persistent, might be worth trying.
Equally, if the player is going to be launching these things, then if it's too fast and too accurate it'll kill things too easily and remove a lot of the fun that way. Price and TL can be used to check these things a bit but shouldn't be relied on: money is little or no object for a lot of players, and neither is skipping a few dozen light years to pick up another batch of bangsticks. Generally speaking, for a big punch weapon, the player should have to use real skill in scoring a hit. If it's something players will use, more than be used against them, then I'd make it slow and almost no homing ability at all: dumb-fire, even. You point it and shoot it and it goes in a straight line until it hits something.
This second option sound more interesting to me, I think: a big slug to punt out at the Thargoid capital ships in the hope of taking them out. A bug-slug. As to why it has no homing ability, if this option is preferred, the handwavium could be that the Navy fear further advances in Thargoid ECM technology – so they've made a missile which can't be ECM'd because there's no electronic guidance system to fool. The guidance system is the hand and the eye of the poor sap pilots who have to lob these things at the onrushing Thargoid battlecruisers, through a hail of plasma and slicing green death ...
Whatever you choose, though, it should be because it's more fun. The logical explanations for it all can be worked out afterwards.
or a jelly baby!! 
