The Jumpgate level (and the second jumpgate-related level later on) are a bit harder than usual. A really key thing to keep in mind is that you can sell things at the last minute in order to reach the power and money goals of a level. In the case of powering a jumpgate, if you time it right, you can sell everything but the power sources and end up with just enough power (and no non-power expansions to drain it away) to finish the level.
Similarly, when you build big new power supplies, turn them off until you really need them at the end of the level.
There are a bunch of other useful tips in this thread, as well: http://www.ninjabee.com/forums/viewtopic.php?t=15