-- Last Updated: Sep-20-12 10:07 AM EST --
You can find a decent one quite cheap if you look aggressively. Like in the hundreds.
But... are you talking about taking out friends who are not likely to be accustomed to waves etc? If you plan to try that, can you get a panicked person back into their boat in those conditions?
I've had issues with people who are freaked out in flat water rescues, sometimes finding it takes two people to get that one back in because they really are not able to follow instructions. I'd think carefully about the environments in which you might be responsible for others.