Considering the fact that I sold Senna
for some 12-13 billion and her skills and SP numbers, and that I sold Jesmine
for 18.5 billion at 55million SP's and Amarr-only skills, I believe it safe to say that Garrick is worth at least 18 billion for his SP numbers alone. I surely would not ask for any less for a 56 million SP toon no matter what he does. Add to that his capital skills, his extensive ship skills, the fact that he has a huge range of other skills even if many, like the industry skills, are barely trained up at all (you save yourself the cost of the books buying this toon) and you have one that's worth loads more. I'd say at least 22-25 billion, likely a bit more. Any deficiency in skills related to PVP seems to be minor and easy fixes inside a couple months of skill-training.
As for your 18 million SP toon good for trading and industry, I'm not sure exactly, but I feel he's worth least 5 billion, nothing less. Likely a lot more, but that's a minimum *I* would never go under.
Then again I've only ever sold two toons and one was mostly a act of "I wanna get rid of this ASAP" so I likely sold it for a bit less than it was worth, so take my advice with a grain of salt...