Hmmm
Well depends what you want...
Something like a portrait lens - 35mm f1.8 nikon (170ukp from jessops)
I tried a samyang 14mm y'day - its not bad around the 250ukp (both of those I believe are new) or a fisheye samyang 8mm. The samyangs are fully manual btw.
If budgets are getting very tight, perhaps look at the m42 adapter (recommend the one with glass) and a selection of older lenses - anywhere between 5ukp and 150ukp.
If you are going to the safari meet on sunday I may be going (depending on weather) and you can try a few of my lenses.
Tokina 11-16mm is great - dropped mine on camera from bag on Wednesday :/

requires a bit of a fix now...
Easiest thing is to use the lens you have and see which focal range you use a lot, then try to find a prime at that range to compare shots with. Ease of use and iq etc need to be compared to your current lens(es).
Good luck on the hunt