It depends on what plants you want to keep and how you are prepared to provide for them. Some plants, like java fern, are considered easy and require very little maintenance. Other plants are more needy and require lots of light and nutrients. If you give plants what they need then they will grow.
As for expense, well to fill a 100g tank outright would cost a good few bucks. Buying a few common stems is just a couple of dollars. Price varies between species, just like fish. The good thing is that not only do they look better, you can propogate live plants so they end up paying for themselves if you can sell back cuttings to your LFS.