Recently Candice and I went on vacation to Bulgaria. When there, I picked up a few prepaid sim cards so as to be able to use some of my devices without incurring expensive roaming charges.
Globul was the network which made the process of purchasing the SIMs the easiest, and the rates were in my view very reasonable. For Lev 9.90 (£3.98 / $6.42) I purchased a prepaid data only SIM with a 2GB data allowance. Even though the SIM was meant for data only devices (tablets, laptops, mifi etc) it worked fine in my OnePlus One, so I was able to engage in the usual social networking and play Ingress too.

If you plan on visiting Bulgaria and need a SIM, check out Globul and consider their network, I personally recommend them. The Globul network is also available in English : http://www.globul.bg/eng/