There aren't experience points, but there are levels, which for the most part are based on your total worth. In SP, you can only move up to certain levels when you complete a SP mission.

Can you get past level 18 in single player? I am having trouble. In my status screen it says that my next level requirement is lower that my current worth. What does that mean?

Yes sure, but can i also level up when I complete a non-storyline (random generated) mission ?

In single player, sometimes you need to increase your worth to level up, sometimes you have to complete a storyline mission to increase. It will tell you in your character status window. It will either give a dollar amount you need or it will say 'complete mission'.

As for having more money than the level up amount. You may need to spend a bunch of money to go below the amount, then earn a little more to go past it. I'm not worrying too much because I figure any wierd thing I run into has been encountered by the beta testers and fixed.

Let's get those missiles ready to destroy the universe!!