Psr-300
Answering this question and staying within your limits must come with some cautions. Many agencies out there are moving to 'digital' transmissions. If you want to monitor one of them, you'll need a digital scanner. It is also worth noting that there are some agencies which can not be monitored by ANY scanner.
To keep within your budget, I suggest the PSR-300 hand-held scanner. I bought mine from ScannerMaster (see link at top of page) for $149 which included shipping and the AC adapter. But the scanner is probably not the only thing you are going to need to purchase. Other items include: a programming cable, programming software, rechargeable batteries, possible alternate antennas, external battery charger.
Certainly if you are going to use the unit at home, or in a vehicle, and NEVER portable; then the PSR-400 base model is good (it is the base version of the PSR-300). But as soon as you want to carry it with you or run it from batteries such as during a power outage, you'll want the '300. I have an outdoor antenna at home and a magnetic mount antenna on my truck.
If you want to monitor an agency which is digital, then I suggest the PSR-500. Unfortunately just the scanner will cost more than $400 unless you wait for a sale.
The PSR-300/PSR-400 are organized as 10 banks of 100 channels each for a total of 1000 channels. This is a lot easier to learn how to program and operate, but often "wastes" memory. The PSR-500 (or PSR-600 base model) use "object oriented programming" and are much more difficult to learn, but can be more flexible to use with the myriad of complex systems out there we want to monitor.
Good luck and welcome to RadioReference.com!!!