Each level of skill is limited by player's level + 1. (i.e. If player level is 5, can level up a skill up to level 6 paying with $CELDA or Dust).
The $CELDA and Dust cost to upgrade skills increase by 100 per level (subject to change based on players feedback during the Beta Season).
There's a logic behind the skills trees that intend to balance the game. To level up a skill beyond level 5, players must have at least 1 skill of every tree at level 5. This limit applies at level 5, 10, 15 and so on every 5 levels.