The reason most "kits" come with a 18-50 and a 70-300 lens is that those 2 lenses together cover a long range, and they are reasonably cheap. The 18-50 is what is known as a general purpose walking around lens, and the 70-300 is a long telephoto, typically used for bringing far away objects closer. Often used for sports and wildlife, but the kit telephoto lens is too slow to really be good at that. Pros usually use lenses like a fast f2.8 70-200 or 80-200 for those applications, or even longer specialty telephotos that cost a fortune (like $5-10K each).

But, these two kit zoom lenses have some serious disadvantages. The Kit lenses typically have an f5.6 maximum aperture at all but their shortest focal length, which means that they are slow, and cannot blur the background as well as faster lenses can. They are also generally not well made enough to withstand the rigors of professional use. And finally, they are not particularly useful for wedding photography or portraiture, both for those 2 reasons, and because you would find yourself having to switch lenses too frequently, as when shooting a wedding, you would frequently need to move between a fiarly wide length of around 24-30mm, and a portrait length of 60-85mm. Both Anbesol and I recommended one better, faster zoom that spanned that range, because if you'll be shooting weddings, you'll need to cover that range quickly without changing lenses.

The 50mm f1.8 lens is a no brainer because it makes a fine portraiture lens, it has a great deal of speed to blur backgrounds and home in on the subject, and they are all inexpensive and high quality. But if you shoot weddings and parties, you'll need that fast moderate wide to moderate tele zoom.

The issue about lenses that span the whole product line is complex, because both Nikon and Canon make cameras with different sensor sizes. The ones that are what is called FX or Full Frame, are very expensive top of the line pro gear. Generally speaking, a good lens - especially at the wide angle end, that covers the whole frame will be very expensive. Those lenses work on smaller sensor cameras, but you will be paying a premium for them. The GOOD thing about using those lenses on small sensor cameras is that the small sensor only uses the sweet spot of the lens. Both of the expensive zooms that Anbesol and I recommended happen to be full frame zooms that could go with you to the full frame bodies. The cheaper kit zoom lenses do not span the larger frame size, and they won't work if you move up to full frame. The Nikon D90 and D300, and the Canon 40D and 50D are NOT full frame cameras. If you DO make a move to full frame eventually, you will find that your lenses all just either became wider-angles, or they became unusable, depending on whether the lens can span that bigger frame or not.

There is no general rule of thumb as to whether Nikon or Canon lenses are cheaper - they tend to be very similar - but, full frame zooms cost more (especially at the wide end), faster maximum apertures cost more, and built in image stabilization costs more. A true apples to comparison between the brands would show that they are generally similar. If one brand's lens costs noticeably more than the other, it's generally because it has one or more of these features that the other one lacks.

One thing I didn't mention, but Anbesol did, is the 3rd party brands, like Tokina, Sigma, and Tamron. They all make some very good lenses, and also some cheap junk. It's tough to give you any simple guidelines with them, other than they are generally not build to the same standard of ruggedness of the BEST, MOST COSTLY Nikon and Canon lenses. But, the better 3rd party brand lenses tend to be better than the Nikon and Canon cheap kit lenses. I don't have any experience with the Tamron lens Anbesol recommended, except that it has a good reputation as one of the better 3rd party zooms.