How can you buy jets that you need, and still have money to buy turrets?

You buy them using sub-prime government subsidized loans

If you "need" jets, then you're prob hitting someone or warring. If you're hitting someone, aim for a target who'll fork over cash when you grab em, then figure you'll be ditching a chunk of debt for the jets you lose during the attack, and with the treasure you gain the ability to purchase turrets post-strike. Be sure to consider your building any new land vs buying up defense, as that will be the primary focus of your cash.

If you are warring and need jets, but want turrets, you are not dedicating yourself to offense or defense. It will cost jets+/oil+/food+/cash+/turns+ to be properly offensive to your opponent. It takes turrets+/cash+/few turns- to turtle up and wait em out. Buying 50/50 is like throwing some punches, then covering up with your gloves. You go all jets it's for the knockout. You go all turrets and you can gas em out while they spend more resources and time working you over. You are doomed to stalemate or worse a significant or fatal counter strike, which is a big fizzle to an epic battle.