I use a setup similar to the one suggested by Maine Sail and tommays. It's works well for me.
A 30w to 40w panel (mono-crystalline for the best efficiency) is about $100 through eBay or Amazon. The Genasun MPPT controller is about $60 or $70. Mounting hardware actually ends up being the most expensive thing to sort out. On a tiller boat a cheap option is to use some good quality clamps mounted at the midpoint on the panel, with the panel mounted on the back rail. That works best with smaller (lighter) panels. It's not meant to illustrate it, but this is how I had the panel installed on my Catalina 25:
Clamps are visible here in the upper left:
That Catalina 25 doesn't have shore power and it's batteries are primarily charged by the 20 watt panel.
On a wheel boat this doesn't work so well because the panel intrudes into the cockpit space right about where the helms person wants to sit. Maine Sail has a guide on how he mounts panels that fixes that issue, but requires over $100 in hardware:
Installing A Small Marine Solar System Photo Gallery by Compass Marine at pbase.com
That is what I've done on my Pearson 28-2 (not yet wired when this was taken):
That is a 30 watt panel and it puts out a little over 2 amps at noon on a sunny day.
Adding a battery monitor system (http://www.pbase.com/mainecruising/battery_monitor
) will give you a lot of insight into your power consumption. I didn't have one on the Catalina 25 and was flying blind but with way more battery capacity (2 group 24, so 150 amp hours) than I needed for an all LED boat. I added one to my Pearson and it's really helpful to understand power demands and battery state.