First of all go to the your card collection and see which instrument cards you hold much. If you have piano lesson cards more than other lessons cards then it's better to buy only piano lesson cards.
everyday crawl in the market and find some cards which fit in your budget.
try to maintain only one type of music lesson instruments cards which will give you more skills per mission by doing the music lessons.
skills is very important to keep ego balanced which you get from fans and missions.
Let's take my example I have
19 R82 RS Vocoder
2 R64 RS30 Guitar synth
I'm holding total 21 rare cards of production lesson instruments and 4 common cards.
my skills reward per mission is 62 to 123.
I'm holding production lesson's card more than other lesson so I'll gather only production lesson's cards to increase skills reward per mission.
1 rare card approximately increase 2.5 to 3 from lower limit and 5 to 6 from upper limit.
here is an example.
My current skill reward is lower limit 62 and upper limit 123 if I'll buy 1 rare card with 50, 60 luck my skill reward limit will be 65 lower and upper 129 or 130.
If I'll add 10 rare card with 60 luck my skill reward limit will be 92 lower and 184 upper.
this a simple calculations for music lesson instruments cards.
if you want to increase your skill reward per mission best way to get more skills buy some instrument cards and increase skills reward per mission.
only buy 1 music lesson cards don't waste your starbits to buy mix music lesson cards
Check F.A.Q to find which cards are fit for your music lessons.