User blog comment:Duff94/Can anyone help me?/@comment-25956839-20170324211549

To answer your question, if you pay to unlock the last tier the next series will unlock.

As is the bottleneck currency, I would recommend you buy the BRL cars and complete the series, without upgrading them. See User blog:RR3 Michael P/My car garage, upgrades and progress though RR3, for how I managed to reach the end of the game (May '15), the tips may help you.