The big differences are in reliability/quality of components, features, and whether there are extra features which may not be used now, but will be useful in the future. You can get a cheap motherboard with less features (less on-board controllers/functions), or a more expensive one with way more features.
Difference of Cheap and More Expensive
Is there any actual difference in performance between cheap motherboards (e.g. ~20 USD) and more expensive models (~100-150 USD)? As my first PC is being build, I want it to be as future-proof as possible, so is it worth spending some extra money on something less future-proof? Will an “expensive” motherboard have better specs or will they all eventually perform the same way when matched with the same processor and RAM?
That’s what you’re paying for, the extras and additional functions. And yes, it is good to buy something that will last, that you won’t have to replace a few years from now when you get your next computer (and get rid of your current one). More expensive motherboards usually use higher quality components (knowing this is an imperfect analogy, but the technology in cheaper ones tends to be “faster” and “better”, while more expensive ones are “slower” and better, with a more balanced architecture using fewer but higher quality components).
Computer manufacturers often sell bare-bones systems which are pretty inexpensive (and perform similarly to other more expensive PCs), so for example if building a PC from parts, they might cost ~$300-$400, depending on which one you pick. But if you were going to buy a computer manufactured by a company such as Dell or HP, most likely it would cost more like $500-$700. So if you really want to get into PC building (I had a lot of fun doing mine) I’d recommend starting with a bare-bones system and not spending too much on the motherboard.
I can’t help you determine how much faster or better your motherboard will be,$50 Motherboard ultimately that depends on the processor and RAM, but you do need to balance your budget between those two. But I would suggest spending as much on the motherboard as you are able to (knowing that too expensive is worse than too cheap) as it seems like it is an important part of the PC.