I decided to take a little break from buying packs and bought a new Mic on RisingStar to help with the music lessons. It cost 4500 starbits which is affordable. I'l l probably buy a couple more Mics in the days to come.
Here's how it improved the XP per singing lesson.
Before the purchase:
Around 12 - 23 XP per lesson.
After the purchase:
14 - 28 XP per lesson.
So the difference is 2 on the low end and 5 on the high end. So given the number of missions per day, it works around 12 to 30 xp per day.
The cheapest Epic card is around 24,000 starbits and I hope to buy one next. Deciding between packs and saving up for an Epic card.