Another advise: if you want to buy an upgrade, but it is too expensive to pay for it at once, you also can buy and upgrade from closest always available ship to your desired ship. For example, you have a $30 P72 as LTI token, you want to have a Sabre Comet, $185, the upgrade would cost $155.
You also can buy an upgrade from the closest lower ship to Sabre Comet. That would be an upgrade from Constellation Taurus, $150 to Sabre Comet, $185, which costs $35.
When you have more funds available, you can buy the upgrade from P72 to Taurus, first upgrade the P72 to Taurus, then upgrade the Taurus to Sabre Comet.