Bali is an interesting place for SIM cards, as you will certainly get a foreigner rate if you are a tourist, which is generally 50-100% more expensive than it can/should be. Sometimes if you're lucky and you find a more off-the-beaten path shop (perhaps not in the center of Kuta or Seminyak, more like in Denpasar or other parts), you can get something closer to normal price.
The last time I went to Bali, I transited a few days in Jakarta first. Buying a SIM card right outside the arrivals area of CGK airport is super easy with the major telcos all represented by official? (official looking at least) kiosks. 3GB + unlimited Youtube for 30 days for IDR 50K (~USD $3.29) Then, when I went to Bali I didn't have to worry about getting a SIM and it all worked fine.
That being said, I was previously able to get something like 2-3GB good for 7 days for IDR 80-100K. Often the packages are much larger I think because it seems many stay for long periods in Bali.