I would say more importantly read the SRB website and you will see that if you do what you are asking you can, and I would most likely say, be charged with a felony.
I would suggest a scanner also as you can program them to whatever you would like as well as not cost as mush as using a two-way radio.
If one knows what they are doing, and thats the largest stumbling block here, monitoring with a radio isn't that big of a deal. Now before the RR police step in and start flaming....
1. Radio must be disabled for transmit, and i don't mean just turning it down in software, i physically disabled the transmit on a couple Astro Spectra's, removing the PA and exciter stages, grounding out the input to the exciter stage, no detectable rf output with testing equipment.
2. You will be limited in your scanning ability... A scanner is so much better, but if you need to monitor a handful of talkgroups then it works good. But you can't scan a ton of talkgroups. You could conventional scan the voice frequencies and basically just have a basic scanner that doesn't require any trickery other than putting in some receive frequencies, no trunk track, if you didn't mind that, but trunk tracking with a real radio is more complicated.
As for Cost.... A good used P25 scanner is in the $300-$400 range. I can pick up an Astro Spectra for $100 or even less. The last two i had i got free for the asking. However, you have to have the programming equipment/software and knowledge to make it all work, and that cost can exceed the savings. In my case i made due with what i had until a good deal on a used scanner came along.
We will see this question more and more because of all the cheap used radios flooding the market.