That article does a pretty good explanation of what TLC NAND based SSDs are doing. With the 500GB and 1TB versions of the 850 EVO, the SLC cache is big enough where it never really empties. If I remember right, there are also enough channels so that if it were to empty there really isn't much of a drop. The drop with the smaller drives is pretty big though:
Typically, the higher the capacity, the smaller the drop in performance. Though in the case of the Crucial BX200, its pretty big no matter the capacity.
Keep in mind though that you'd need to be doing a lot of writes to fill up that SLC cache. For the average consumer, this probably isn't going to happen. And even if it does, you'd have to be writing say a file that was 5-12GB in size (depending on the capacity and/or cache size).
If you're looking at a sub-$50 drive, you're probably going to be limited to ones that are 250GB or less. If thats okay, it may be worth paying an extra $10-15 and get one with MLC NAND. The difference in price isn't that much at that capacity.