how much should I pay for a RJ-11 duplexor?

While at the brand-new Super Wal-Mart, I bought a phone jack duplexor for $4.24 plus tax. A Belkin one is listed at $0.88 at HP's website with $5 shipping, while ~$6 at RadioShack's website. I need it to have my desk phone work again whose jack has been recently taken by my new VoIP adapter. wonder why my VoIP vendor doesn't supply a duplexor. Liability concern ? Techies I know all plug their VoIP adapter's FXO port to a regular phone jack in the house thus all the jacks in the house are hot again, after disconnecting the regular phone line off the hook at the curb.