Tier Increase Rate
Tier increase rates can be found on items. They are thought to affect how fast an item tiers up.
Not enough research has been done to determine exactly how they work. However, they can be thought of as affecting the overall cost to tier up an item.
The following is based on my personal ideas and have not been verified or validated by any research. The best way to illustrate how tier rates may work can be done through an example. For example say we have two guns at tier 0.0 and both have tier rate for level I at 130. I think they would both cost the same amount to get to tier 0.9. However, each one will increase at seemingly random rates. One may tier up really fast to tier 0.7 then take much longer to tier to 0.9. The other may take considerably longer at lower levels and tier up faster at the end to get to 0.9. Both would I think may cost the same amount to get to tier 0.9.
more info needed