The Bulgarian split squat is a great single-leg squat variation to build strength and size in your lower body and also address imbalances between the left leg and the right leg. It will show its strength the more load you can lift. In our training we use it as a squat variation and we also do singles.
In order to be able to progress the load, you need to have a stable base on the ground. Too often you see people not having this stable base, which leads to:
Start from the bottom up. This helps you to get comfortable with the bottom position. Once you're comfortable with the bottom position and have consolidated your movement pattern, the weights you'll be able to lift will follow.
As a rule of thumb, I've found that my athletes are able to use 80-90% of their back squat weight for the same amount of reps. For example, if they can do 3 reps of back squat at 200 kg, they can do 3 reps of 160-180 kg in the Bulgarian split squat.
No, you won't be able to achieve the same depth in the Bulgarian split squat as in the back squat. However, if you standardize the back squat depth and the Bulgarian split squat depth, it's a valid relation.
So, get the technique right first, get comfortable with the bottom position, and you'll soon be able to load up the weight.