100x
eco is always trump.
Generally coins work different than in real life and cost is judged different.
In real life you might think "I have XY money on bank account so my next acquisition should cost max so-and-son."
In the videogame zero-K coins come in and coins go, in constant stream. You can always buy everything, now matter how much coins in bank. Like deferred payment. (Ratenzahlung)
Ideally you never have 3500 coins saved in storage but instead are spending coins as fast as they come in. To judge cost look at the displayed estimated buildtime.
3500 buys handful main-spam-units of each factory.
For example 14x
3500 worth of random mixed shields makes a ball-army that keeps recharging each other.
3500 buys 1 large mech thing. (popular choice)
(strider hub sold seperately)
3500 buys various flying stuff to surprise-one-shot-skill many things like Commanders, striders, big buildings,...