Project CELTICA - Skills

This time I'll write about skills a bit. Well, I mostly picked the "common" skills for an RPG, and I tried to tame it as much as I could to the system I had in my mind.
I am imagining the skills in a 5-scale system, and to improve to a new rank, one has to pay for the previous ranks too from his/her skill points.
Making a skill check is a d12 + dx + Ability bonus roll, "x" means the die given by the skill level.
So it would go like this:

• Beginner - d4 - costs 1 SP
• Apprentice - d6 - costs 2 SP (3 SP total)
• Journeyman - d8 - costs 3 SP (6 SP total)
• Expert - d10 - costs 4 SP (10 SP total)
• Master - d12 - costs 5 SP (15 SP total)

At character creation I'm planning not to let the players take skills above Apprentice level. The amount of skill points a character gets in the beginning and per level is still a think to be thought over, I would like to make the skill list first. 

A skill can't be improved more than one level at a level up. An exception is when a character decides to sacrifice 5-10 years of his to learn the trade, e.g. Blacksmithing. Then he can improve his skill more than a rank, but in the meantime, ha mostly focuses on learning the trade rather than chasing monsters and treasures.

About the DC-s of the checks I would set the following:

Easy - 8, Medium - 12, Hard - 15, Challenging - 18, Heroic - 22.

My method setting these numbers was to allow a character "skilled on that DC level" to have a good chance to succeed on his roll. So, for instance a Rogue is an apprentice in picking locks. He is trying to pick an ordinary lock, which is a  DC 12 check. With a Dexterity bonus of +2, his roll is d12 + d6 + 2, so on average this roll would be a 12. 

So these are the plans for the skills, I'll post the skill list sometime later, I still have to think them through. Next time I'm going to describe some general ideas I have in mind for the game, just as I have them written down in my little notes.