Now, I would be very very surprised if this hasn't been covered before, so I'm sorry if I'm kicking a dead fish.

Here's the thing:

In both TSoY and the new Solar System booklet, buyoffs are always 10 XPs. Even if you use a standard advance of more than 5 XPs.

Why is that? Just for simplicity? Because it sounds dangerous.

For example, if your character has a change of heart, and buys off the Key of Bloodlust, replacing it with the Key of Pacifism, you're like "Yeah! Cool!" You get 10 for the buyoff, spend 5 on the new Key, and have 5 XPs gained for further fun--because you made a cool change to your character.

But if your standard advance is 20 XPs, then, theoretically, that maneuver would either be impossible or it would actually COST you 15 XPs, only to be back to where you started (you're just swapping one Key for another). That seems like it goes against the basic design of the system: character change is rewarded in the Solar System, not punished!

Also, having a buyoff that is greater than a single advance means that you can never run into a "dead zone": no matter how many Keys you buy off, you always have enough XPs to buy another. If your advance is larger than your buyoff, then you could buy off your Keys and be unable to replace them with others, leaving your character without any Keys, and no way to gain experience.

So, what's up with this? Is it just a simplification? It seems like setting the buyoff at twice the standard advance, or the standard advance + 5, would be more sensible.

The reasoning here actually concerns the overall motivations of setting your Advance cost at different values at all. Consider:

5xp: When the Advance costs 5 xp, you're actually making a profit from swapping Keys. All well and good, as the reason to set your Advance cost to this low value in the first place was the fact that you want characters to change in a quick and dramatic manner. Profit from Key swapping supports this nicely, encouraging players to keep an eye on good ways to have their character change.
10xp: At this point swapping a Key is an even deal, which means that it's not actively rewarded. Players can make the choice of changing or not changing their characters based on character depiction concerns solely.
15xp: Now you're starting to actually lose xp, but that's OK, because why else did you set the Advance cost this high, if not to slow down character change. The player will need to think twice before he switches Keys around - he will only do it when he genuinely wants to change the way he plays the character.

As you can see, the constant buyoff cost of Keys interacts with the Advance cost to either encourage or discourage Key switching, which helps to emphasize the chosen character of the game: if you want a leisurely, considered and deep pace for the game, you'll want Advances to be expensive, but you'll also want the thematic change of characters to slow down by the same degree. If the buyoff reward went up with the Advance cost, we'd have a situation where it's very expensive to make your character grow, but very cheap to have him change laterally, by switching Keys.

As for the practical concern, it's not actually possible to end up totally Key-less, as you can still gain xp from Key Elements introduced by the SG. Of course that'll be a slow process, but it's the price of getting rid of all of your Keys. (Also remember that gaining xp slowly is not a problem, as long as the player makes that choice consciously and not by mistake.)

Finally, if the above reasoning does not work for you, feel free to mess around with the Key buyoff independently of the Advance cost. The basic logic there would be that the higher you set the buyoff in relation to the Advance cost, the more rewarding it is to grab any opportunity to change your character around. I've never actually played with a different buyoff myself, but I imagine that it won't harm the game too much as long as you don't go overboard in rewarding buyoff - there's some danger of devaluing normal xp rewards if you end up giving more than five or so xp of profit per Key buyoff, leading to a situation where a character might actually gain xp faster by simply cycling Keys. Going the other way is potentially more reasonable - I could see some curiousity value in a game with a 5/5 spread, for instance, depending on the group. 15/15 would also be conceivable with some people I've played with. I doubt that I'd bother myself, but whatever helps others sleep at night is fine with me.

Thank, Eero.

You have a real knack for explaining the way all the parts of the Solar System work together. I think another piece just clicked into place for me.