Alligator,
Yep...they're expensive!!! Depending on the wattage of the panel, prices can range from $12 / watt to as low as $4 / watt. A typical 100 W solar panel will run $550 -$600 each - wholesale, uninstalled. I've seen a few for just a little less on E-bay, but don't be fooled, not all solar panels are equal. Make sure to buy a panel with highly efficient photovoltaic cells. Some of the newer panel technology continues to produce impressive amount of power even on cloudy days. Some panels ( typicially those that have very small rectangular cell arrays covering the whole surface of the panel) drop off very fast during less than perfect sunlight conditions and angles. While not always the case, the larger semi circle or full circle cells tend to be the higher output producers in less than ideal conditions.
As far as installation costs, solar panels are typicall easy to install on a RV. Two panels installed should not be more than two hours labor. You will need a power controller that controls the amount of power from the panel to the batteries. A two hunded watt controller can be pricey, but can be had for around $150 or less. A typical highly quality honda 2000 watt generator can be purchased for what many people pay for 100 Watt solar panel installed. That said, over the life of the panel it, will cost less and produce more power than the generator will during its life. If you're a typicall weekend camping warrior, the Solar panel is not the best investment. If you're a full timer, then I would highly consider the solar alternative.
Good luck.