What is the real difference between these two bits:
They are both carbide, both ball-end for 3D carving, but one of them is $3.50 and the other is more like $40. Is there something I’m missing about that expensive one that makes it cost 10x as much?
Not really sure why the 10X price difference in those two particular bits. But in general the price of the bit is mostly determined by the brand name it carries. A solid carbide bit from a brand name manufacturer will be in the $40 range, Where the same carbide bit from a Chinese seller on ebay will be about $3.00.
The reason is that the bit from the name brand manufacturer has a huge amount of quality control behind it. You know the size is correct and the material in the bit is exactly what was advertised. So a solid carbide bit will be solid carbide. And you can know almost exactly how long that bit will last for your production requirements.
If you are using bits for large production runs then that type of quality is required, you just cannot deal with bits that are not the exact size and design you expect.
For hobbiest (at least in my case) I have been very satisfied with the cheap Chinese bits I get on eBay. They do vary in diameter, so a 3 mm bit may actually be 2.8mm. So I measure each bit with a caliper and record the actual size. With the actual size entered into Vcarve they do a great job. Since they are about a tenth the cost of name brand bits I am not very worried if they only last half as long. But with the amount of cutting I do (about 10 hours a week) I have not had one wear out yet. The low cost also means I can afford to buy a large selection of different diameters and types which I find very helpful since I never know what project I will be doing next.
The prices he quotes are for 1 of each.