I roller skate (indoors) about once a week. I justify the cost ($6) because I wouldn't think twice about meeting a friend after work once a week for a cocktail, which would be about the same if not more.
*Be careful* when you roller skate outdoors. Most skate wheels were designed to be used on smooth indoor rinks. The problem with quad skates (instead of inline ones) is that you are more likely to fall if you hit a rock, because more surface area is covered by the wheels. If you look at the front of roller skates, you have two wide wheels and a space, allowing all kinds of ruckus to happen if you hit a rock, twig, or bump in pavement. If you look at the front of roller blades, you have thin wheels that taper towards the ground, which allows for rocks to get nudged out of the way.
I'd recommend buying outdoor wheels, if you can. Also, grab some protective gear for your wrists and head.