I think the FZ18 takes the same battery as the FZ7, FZ8, FZ30, FZ50, and others. I have an FZ50 and bought an off-brand battery from Ebay for about $10 including the shipping and if it's any worse than the factory battery, I don't notice it. There are plenty of third-party batteries available for a lot less than $70, that's for sure. Normally I am a beliiever in 'penny wise pound foolish' but there just can't be that much so special about the Panny battery that it should cost seven times as much as what I paid for an off-brand.
Some people claim the third-party batteries don't hold a charge as long, and perhaps some of them don't but frankly I don't notice any difference. Don't necessarily buy the very cheapest battery you can find, but believe me there are plenty of good ones that are not anywhere near $70!