I would go with the ACK-450 which is readily available through ACS, Chief, etc.
Like all economy ELT's, they are only 121.5/243, but it does have voice capability, and runs on D cell batteries.
ACK is currently certifying this ELT. It is not under $500, but IS under $600. Check it out on Aircraft Spruce or on the ACK web site. They aren't selling them until certification is complete. Don't know how much of a hurry you are in.
BTW, it will be interesting to see if the other manufacturers bring their prices down once ACK starts selling these.