Master Lock Hardened Steel U-Lock
U-locks are best for the city. And shorthand is, the higher the price, the better the security, but if $40 is still a bit of a stretch and you still want something semi-secure, then Master Lock's basic u-lock will still provide a strong deterrent. It doesn't come with the same kind of warranties as do the OnGuard or Kryptonite locks, but it's only $25 and it keeps your bike safe and locked up. That's what you want. It comes with two spare keys as well as a mounting bracket, so transportation is a breeze.