gree,

Check the parts list. The Dotan stocks and rudders will be about $1K or better for the pair. A new sail may be $800. All will add up fast. Conversely, I have several Hobie rudder assemblies from dead boats, will they be as easy for a ten year old to operate as Dotans? No, but they're "almost" free. Some searching the windsurf clubs may net a suitable sail for next to nothing as well.

You're right, it could be built cheap.