Good and cheap are oxymorons when it comes to bike locks, as are good and lightweight.
You should look into the used market (Craigslist or ebay or other such place) for a good used lock sold cheaply.
For good and light, I might suggest one of those newfangled titanium locks such as TiGR, but they run over $150 and are keyed. Perhaps you can find one used...
If you actually find a new, cheap, lightweight and highly secure lock many of us would be very interested.
Good luck.