We'll need to know a little bit more before we can advise. . Do you want a portable or a home base scanner?
I see from one of you other posts that you are from Houston, TX Harris County. You will need an analog trunking scanner to hear many of the agencies there. At least 1 is digital, but most are not. I think you will be hard pressed to find a new analog trunking scanner for $150, but used might be OK. Here's a list that includes some older models that you might find on sale or used: http://wiki.radioreference.com/index.php/Category:Analog_Trunktrackers One example would be http://wiki.radioreference.com/index.php/BC246T At the bottom, you will see the eBay selection for this model. It is a good one, and you might be able to get it for around $150. Try clicking on the other models in the list to see what they are, and what eBay listings might be. Some of the older Radio Shack (PRO-xxx) might also be available.
Programming most trunking scanners is a pain, partly because of Talk Group lists. You should consider getting one that accepts programming from you computer, If you click on the models, you will see a description of their computer control capabilities. Software runs around $30-$40, unless you get a Uniden, for which there is a free program. The software will simply your tasks, and make it easier to understand all the features of your scanner.