Obviously I can tell Kaidan was based off of him but their faces have a totally different essence which makes them look like 2 totally different people. It's hard to explain.
I think I can try to.
I've watched the cosplay panel video, and I really-really wish RS could've joined them. But he was nice to make a record, which is also great.
Costa is like concept-art. I'm not saying this to offend him or his fans, but that's exactly what he is for me. Concept-art is essential for the game, it expresses some ideas about the character's appearance and nature, it's greatly used by artists and writers, it helps to create the actual content, but it's not really a character himself, and it might not even remotely look like a final in-game character.
Most of the time he did not look like Kaidan at all (to me at least). Eyes, random expressions - there were something 'kaidanish', sometimes. I'm glad he does not seem to exaggerate his role, jokes about how 'he was used'. I'm also glad that he preferred to keep his natural looks instead of making himself 'more like Kaidan'. Looks like a nice guy.