There's really no answer to that.
First of all, if we look at skills. BESM did something quite smart which is have genre based skill costs rather than fixed skill costs, most systems don't even do that, yet they have similar costs. FATE is already like that, so you've already got a situation where the ranks aren't worth the same even if you pay the same. Another thing to consider is that the distribution is along a curve with the dice, so if you're playing a 4dF system high ranks are worth a lot more, but if you're playing a d6-d6 system the roll is worth more. The other thing with skills is as you improve one skill, another goes down, so improvement is just relative. If you drop your +5 skill to +4, that could represent losing some ability, or it could represent everything improving by +1, and just that ability slipping. FATE doesn't have a traditional advancement system, which muddies things even more.
Next you've got Aspects. Aspects are described in words, rather than in terms of game mechanics. They can give a re-roll or a +2 if tagged, with spending a FATE point, and they can be used to generate FATE points. Now the value of a FATE point is rather ambiguous as it's limited, but with a +2 bonus it's also what can help you get a couple extra shifts for some spin. It will again depend on the system you're using. In 4dF, it might give you an extra boost just to be able to succeed, or reliably re-roll a fail. In d6-d6 it's more likely to have an effect on degree of success. Since aspects are described in words, their scope can vary quite widely, and the issue with their scope is they're balanced based on how frequently they can be compelled and how frequently they can be invoked. Since a compel earns you FATE points, a disadvantageous aspect is actually worth quite a bit.
If we're talking about FATE points, their value depends on how the game is played, as well as based on your aspects. If you've got aspects that get lots of compels against you, a FATE point isn't worth much. If you've got aspects that don't get lots of compels, a FATE point is worth a ton. The storyteller style will also affect it, with some storytellers you can ask if something is present, and they'll usually say yes. With others you'll have them asking you to spend a FATE point. Finally, what you also get is that if you trade an aspect for a FATE point, FATE points get more valuable (less chance to compel and earn them), and if you trade a FATE point for an aspect FATE points become less valuable. This would also depend on the type of aspect you're trading. If you're trading a disadvantageous aspect for a FATE point, you're actually giving up quite a bit, because that aspect might be worth 2-3 FATE points per session. If you're trading an advantageous aspect for a FATE point, you're limiting your ability to spend your FATE points, so you're increasing reserve but also decreasing their utility.
Ultimately, the only thing you could really translate is stunts and skills. Stunts are sub-skills (in a way), so you could say a skill rank is worth more, something like 2:1.
Other than that, if you trade in FATE points for aspects, you generate more ability to get FATE points, so you're increasing your aspects AND your FATE points. It's not actually a trade, you're just starting out with less aspects.
The only other way to see the value of a FATE point is if you're using the optional aspect locking rule, in which a FATE point would be worth 2 ranks in a skill.
So you could say 1 FATE point = 2 skill ranks = 4 stunts. You can't mess with aspects at all though, and play style could completely imbalance that - skills have different scope and utlity, and FATE points have different scope and utlity. One skill rank might be worth 1 FATE point depending on the skill it's used for, while another could be worth 3 if it's constantly used. If you made a really poor choice in skill selection, a FATE point could be worth 5 ranks in a skill (particularly in a d6-d6 system).
This really reveals the problems that systems like GURPS run into, where balanced characters point wise are completely imbalanced as far as effectiveness goes. Systems like EABA that restrict the number of points that go into different categories, and moreso BESM, end up smoothing out that balance. Systems like nWoD and D6 ensure balance even more by having a pool of dice for skills and attributes that are spearate from each other and not exchangeable. The way FATE is set up it ensures that each character has elements of everything, is well rounded and not overly specialised. This prevents one character from being useless by being specialised in the wrong way, and prevents another from being overpowered by specialising in the right way.
You can try having some sort of official balance between values of characteristics, but you're better off either leaving it alone or eyeballing it on a case by case basis for what feels right. If someone's character concept requires a couple extra stunts, and they don't rely on aspects so much, you could let them trade that, or one aspect and one FATE point. I'd suggest giving it a 1:1 value with maybe a max of 3 points that can be moved around, only to be used when necessary, rather than something players can work with at a base level to optimise their characters mechanically.
