The 18-55mm zoom already is Nikon's bottom of the line option. And as Fhotoace said, you get a considerable discount on it when you buy it together with a D5000 body. At B&H, this lens costs $120 as part of a D5000 kit vs. $185 for the same lens sold separately.
That's just $120 for a pretty okay lens. You're not going to find anything better for that price...
* A used, non-VR version of Nikon's 18-55mm will set you back roughly $100. So for just $20 more you get VR (Vibration Reduction, i.e. image stabilization) and a warranty. That's a no-brainer.
* A used, cheapie Sigma/ Tamron/ Tokina zoom? Why bother. They're crap. These companies can and do make some good stuff, but not for under $100. In that price bracket they make stuff that squeaks when it focuses, takes forever to focus, provides worse image quality than shooting through a Coke bottle, and snaps in half when you look at it wrong.
If you absolutely, positively need to cut costs, look for a cheaper total package. In that case, forget about the D5000 and get a used Nikon D80/ D60/ ... (with a used 18-55mm non-VR lens.)
You hint at using the camera to provide a professional service, so let's put this in perspective:
* Loan whatever you're short and pay it back when your photography starts generating income. Consider it a business investment. You do want your gear to meet a minimum standard, right?
* The Nikon 17-55mm f/2.8 is what you really want. That's a $1260 lens. The lens that you currently consider too expensive costs 1/10th that price.
In conclusion, order the kit and be kind to ponies. Thanks for reading.