Choice of lenses

Depending on what you want to photograph, will determine your next lens purchase. Some other things to consider besides cost are, weight - will it be too heavy to take with you? Will it mean buying more filters if the filter threads are a different size? Size - will it fit in your camera bag? Does it have VR/IS? Is it compatible with your older cameras - does it miss an aperture ring? Will it need another tripod foot? (Usually only big telephoto lenses). Zoom range - too much/too little?
Do you need a very large aperture lens - today's digital cameras are pretty good at higher ISO.

Having a heavy lens on a lightweight DSLR will affect the balance, which in turn will affect your picture taking experience, and whether you bother to take the picture at all. Consider the overall effect on your system as well. If you plan to travel a lot, or carry your gear a long way, heavy lenses may mean leaving some of them behind. The higher priced zoom lenses are as good as prime lenses and have been for a long time.

Equipment choice is an ongoing process so don't think you can get it perfectly right in one go. "Don't burden yourself with too much gear" if one of my favourite photo sayings.