LW Pro is a one-off payment... if you buy Extended Pro for a year then your benefits (being able to download new Pro installers & email support) will last for a year.
When downloading songs with LW, you're connecting to the Gnutella network & downloading files from other users who are sharing their files. There's no charge per download.

If you haven't used LW before, try the free basic version first. If you're happy with basic then think about buying Pro if you want to
The only official site for downloading the free basic LW, or buying Pro is: Official LimeWire Website & Free Download of Fastest P2P & Torrent Client - Lime Wire

Just remember that buying Pro doesn't make it legal for you to download copyrighted files
Copyright Information - Lime Wire