Is 70 mg of caffeine per serving of JostArriba a lot or little?

Depends on what you are comparing it to:

One cup of 8 oz brewed coffee on average contains 180 mg of caffeine, a single espresso shot contains 75 mg, while an 8 oz cup of decaf coffee contains about 15 mg of caffeine.

Most energy drinks contain 200 mg to 300 mg of caffeine per drink.