Go to a broker [or your own market board], select 'Spell Scroll' for item then pick Grey [or higher] to Orange for your level limits. [Spell Scroll will also show ability upgrades like Rune and Essences]
Then examine what is available and compare with what you have. [At least get Apprentice II level since App2 spells are readily available for COPPER]
You need to bear in mind
Frequency of use
Power requirements
Effect
Frequency of use
Probably not worth upgrading something you hardly use. Duh :) Basically look ahead to the next replacement for the ability you want to upgrade. It can not only be better, but there are certain level limits in the game so an App 1 spell that you just got on level up might be superior to a Master 1 spell you got 10 levels ago and the new spell might work on mobs that were immune to the older spells.
Power Requirements
Mininum Power required. - since this goes up with the level of the ability, you might not want to upgrade so fast till you are over level 10 or so.
Efficiency. - the higher the ability level, the more power efficient it is.
Spells and abilities do not increase in power usage as YOUR skill level increases but the effects can until you have mastered it. So even though power cost remainst constant, the benefits might increase.
Effect
The higher the level of the spell, the better the effect. However the differences between App 1 & Master 1 vary from ability to ability. Sometimes, even an App 2 is already enough.
Cost - Do you really want to spend 15sp on a new Adept 1? Or get the 3pp Master 1 version?