Sao has a point.
The faster you unlock characters and the more characters you unlock, the more credits you need to spend to get them to useful levels. It can become a vicious cycle--you have too many characters so now you need droids and credits, so you buy those but now you need gear, so you need to buy crystals to get the gear. If you go at a slower pace, your credits, droids, and gear tend to stay in line.
You get a couple million spacebucks a week from Galactic War, and can make a good chunk from the later Cantina boards. So just focus on working one person at a time and it gets done pretty fast.