Backpack works in a pinch and it's obviously better served for push ups, one arm push ups, things like that.
A good belt should last you a lifetime. Just visit an old school gym and take a look at the house belt they let members use.
I bought a mediocre belt back in 2010 and the only thing I had to replace was the clip.
As you get stronger, the shoulder straps on a heavily loaded backpack (without waist straps) might interfere with scapular movement. This is just a theory on my part because I primarily use a belt for dips and pulls.
The only time I use a weighted backpack on pull up variation is a movement where you do a dip with one arm and simultaneously pull up with the another; with the torso and legs forming an 'L' position. Unlike traditional weighted pull ups or dips, it's not something I can go really heavy on and I just move better with a pack for that particular exercise. If/when I get strong enough to ditch the pack, I'll have to figure out how to incorporate a belt that doesn't hinder execution.
Another advantage of using a belt is it's a good way to blast the core. It takes practice but once you learn to squeeze the pelvis into posterior tilt and hold it for the set, it forces the rectus, tva, internal/external obliques into an isometric contraction. The tilt doesn't have to be exaggerated at all; point the knees slightly out, squeeze the glutes, feel the lower back go into a neutral position. I wouldn't consider it a main core movement - just a nice benefit to doing weighted pull ups with a belt.
Do not attempt to kip, butterfly or do any laughable swinging with a belt and weight on. If you don't believe me, try to visualize what that would look like. And for the sake of healthy elbows, more isn't better.