$30 is a great deal to be honest.
If you are really that strapped for cash, and you have access to a Wii or Wii U then look into USB loading.
I use USB loading myself, and have never looked back. Access to Melee texture hacks is great, and also I love having both my Project M and Melee stuff all on one drive. If you're looking to play with a Cube though, I would jump at $30 personally. You'll have to be very patient to find a cheaper price, and it will likely not come from online or a store; to get something cheaper you're probably talking garage sales or something where the seller doesn't really know the value. A while back I was able to find a copy at a flea market for $5 in a bin with a whole bunch of old Cube/PS1 games. Lightly scratched but served me well for a few years. That find was largely luck though, as I bought it from a mom who was selling her kids' old stuff.