Skills Revised - Training to be a Wizard

I described my skill system here. In brief:

For every 10 XP you get to spend 1 point on skills.
You start with 100 XP and 10 skill points to spend.

Skills cost 1 / 1 / 2 / 4 / 8 / 16 / 32 / 64... points for 1st / 2nd / 3rd... level.

Thus for example, third level costs you 1+1+2 = 4 points. You don't have to add this up as when you cross them off:

1 / 1 / 2 / 4 / 8 / 16 / 32 / 64...

The total cost is the first number not crossed off.

Spells cost you twice as much as other skills - i.e. 2 / 2 / 4 / 8 etc.

This means that whatever decisions you make early on don't have a big impact later on - if you decide that spending one point in athletics was a mistake then in time that measly one point becomes unimportant.
Secondly it doesn't matter when you spend your points on your skills - that first level in athletics always costs 1 point no matter when you spend it.

Together this means that the metagame of skill choice is downplayed - choose what you feel like and stop worrying about the "best choice".

We've been playing with this system for about two years now and the characters have maxed out at tenth level in some skills, and this has laid bare a known quirk in the design - due to the low relative cost and high usefulness of spells all characters can now cast some spells.

When it comes to a Wizard taking a couple of levels in fighting skills that's fine  - this is precisely what the rules were designed to encourage - but a Fighter suddenly learning some spells seems wrong. But I couldn't see how to fix this without breaking the system.

Then I realised I could replace the "spells cost you twice as much" rule with the following:

If you learn spells then you only get 1 skill point per 20 XP.

If you only take fighting skills, then this is the same as before.
If you only take spell skills, then this is the same as before.
A Wizard who starts learning fighting skills can still do so, they just cost more (as do all skills) - so a slight change (in effect this is just a -1 penalty on all non spells skills).

BUT...

If a Fighter who has spent 100 skill points (1000 XP) wants to learn one spell list to first level this doesn't cost 2 (as in the old system) - they have to get another 1020 XP before they have 101 skill points and spell ability.

In game the explanation is simple - they have to train to become a Wizard - and the longer you leave this the more difficult the training.

So with this system (which I'm not imposing retrospectively on the current campaign!) it makes sense to have pure Wizards, Wizards with a bit of skill at fighting, and also true Fighter - Wizards, but not Fighters with just one or two spells.