Well you can certainly spend more money on glass if you'd like, but a lot depends on what your using it for as to whether or not it would really be a good investment for you. If your just looking to get sharp images and your working with enough light really your best bet is to stop down the lens and up your shutter speed.
If you want really razor sharp images, try upping your shutter speed to 3x to 4x the focal length your shooting at and stop the the lens to somewhere between F8 and F11. Assuming you have enough light you can get some really remarkably sharp images that way. Just keep an eye on your ISO and don't let it get to out of control, the upper end limit on ISO will depend a lot on your camera body and your own tolerance for what you believe is an "acceptable" amount of noise.
If you find some of your lenses are "soft" in the corners then shoot a little wider than what you actually need and crop out the softer portions of the photo.
Primes are great, don't get me wrong. I use a 50 mm 1.8 AFS-G and an 85 mm 1.8 AFS-G quite a bit, and I love them both. But in my case I do often find myself in situations where I need the faster glass because I'm shooting in situations where there isn't enough light and I can't use a flash. In those cases, when possible try to back up a little bit and shoot wide - the further you are away from the subject the greater the DOF (Depth of Field) will be and the more likely it is that your subject, all of your subject, will be in sharp focus.
When your shooting at shorter focal lengths like 35 mm or 50 mm and you need to shoot with the lens wide open or close to it, say 1.8 - well on a 35 mm if you are 4 feet away from your subject your depth of field is only about 5 inches. Back up to 8 feet and you increase your depth of field to roughly 1 foot, 4 inches which makes a huge difference. So I usually recommend if you have to shoot wide open, back up a few feet and shoot wide, don't try to compose the shot as you'd like it on camera or odds are good your going to wind up with too much fall off and a photo that looks too soft as a result.
So my recommendation would be to save your money for now unless you find that you really need a lens that will work in lower light than your current 18-140 in something longer than the 35 mm focal length you currently have.
Just my two cents worth of course, YMMV