That would have to be my biggest complaint.

I've read on other boards that the main plot line ends before you achieve level 20. And there is oh so much good stuff out there for the higher levels when all you do is fly around and goof off.

Yup pretty much. That would be my principal complaint as well. It is impossible to reach a level higher than 18 before beating the plot. After you do there are lots of places to explore and quite a large number of ships to fly. The problem is that it woudl be nice to fly those ships before the end of the plot. It seems unrealistic to be flying around with 800,000 credits and not be able to to do anything with them. I think the level requirements for some of the ships like the stiletto should be lowered.

I'm currently at lv 18

I have around 160,000 $ and the next level requirement says 12,000 $

???

maybe if you get enough money you can go a vacation with those "Spa and Cruise" guys

Does that req mean I need 12,000 more?

I thought that was the target amount to up a level....

It means 12,000 left before you next level
It changes to that system after you beat the SP campaign