Negative stats, such as negative critical resistance on Treadfast amulet, cost double less sink as long as they're negative. Moment you try to turn negative to positive it goes back to it's normal sink value. So, once you try to reach 0+ (in this case critical resistance) runes sink goes back to it's normal (from 1 back to 2 sink). Not just for this item, but any item. Once you try to do -1>0 runes sink is being on it's maximum.
Also, not sure if I understood your sink calculating, but if some rune (example: Range- 51 sink) drops you'll gain as much sink as that rune has. So not 40, not 50, but 51. It's the same for every single rune.
And as Reaper said, your sink on runes is terribly wrong. Some are correct, but a lot of them aren't. Here's a list I used to use and a list I used to update each time they changed something (within years Critical Hits sink, Heals sink and now, most recently, Vitality sink):