ToC and CoC both have tables that give "this is roughly what this should cost" so waiting until you have the rulebook in front of you is probably the best bet. However, I don't know about ToC, but published CoC adventures are often wildly inconsistent (both between adventures and internally) as to what particular SAN costs are.
When I'm setting a SAN cost, the general rule to go by is that 5 points is the minimum needed to possibly trigger temporary insanity. So anything where there should be any possibility of that happen should be about 0/1d6 or 1/1d6. Anything that would be disturbing but not cause a chance of mental breakdown should be 0/1d4 or 1/1d4 or less. Anything where it's going to be disturbing to anyone should have a loss on a successful role of 1, anything that could be plausibly shrugged off with no effect would have 0.
So, for example, seeing an image that is fairly disturbing but could be shrugged off with no harm might be 0/1d3. Seeing a monster that is clearly outside of normal human experience should probably be 1/1d6 - monsters are often statted as 0/1d6, but I usually raise it to 1/1d6. If a monster is something someone has seen before, and isn't attacking them (eg a byakhee you summoned to ride, or a ghoul that just wants to be friends), I usually knock it down to 0/1d3 or 1d4. Anything above 1d6 loss gets progressively nastier. Only really heavy-duty stuff should involve multiple dice, or should have a chance of 5 or higher SAN loss on a successful roll.
Plus, consider dice probabilities. 1d20 has a higher potential result, for example, than 3d6, but 3d6 is going to cluster around the mean average - 1d20 is as likely to give you 1 as 20, while most 3d6 rolls are going to be 8-12.