If you buy a Spartan warbond at 65$ with fresh cash, your target ship to which you will upgrade the spartan from must cost more that the Spartan @80$. Upgrade system takes into account the standard melt value in store credit purchase for a ship. You can not upgrade a warbond Spartan to an Arrow (credit value 75$) or Nomad (credit value 80$) for exemple.
So for exemple, you can buy a Spartan warbond @65$ with LTI, and upgrade it to an Avenger Warlock(80$) for 5$ (85-80=5$) or a Gladius (90$) for 10$ (90-80=10$).
So you save 15$ doing so if you want a Warlock or Gladius in this exemple.
You can cumulate more upgrade, especially warbond upgrades that will be released this IAE if your ship cost a lot, and save more in the process.
I wrote a guide here :
Figured i would write this guide since big sale events are always the time you are thinking about your fleet building and don't want to make any mistake. RSI has already some wonderful FAQs, but sometime you need to dig through them all to get the info you are looking for. Please refer to those...