How much do hydroplanes cost?

That depends heavily on what you're looking for. As with anything, there is a wide range from $10 upwards. For example:

A remote control model - google.com/products/catalog?q=hydroplane+price&rls=com.Microsoft:en-gb:IE-SearchBox&oe=UTF-8&sourceid=ie7&um=1&ie=UTF-8&cid=16206706139610573009&ei=Ogj6SojGD5DymQOz1JG0Cg&sa=X&oi=product_catalog_result&ct=result&resnum=6&ved=0CBcQ8wIwBQ#ps-sellers">http://www.google.com/products/catalog?q=hydroplane+price&rls=com.Microsoft:en-gb:IE-SearchBox&oe=UTF-8&sourceid=ie7&um=1&ie=UTF-8&cid=16206706139610573009&ei=Ogj6SojGD5DymQOz1JG0Cg&sa=X&oi=product_catalog_result&ct=result&resnum=6&ved=0CBcQ8wIwBQ#ps-sellers

$326

A real one: somewhere between $300 and multi-millionaire requirement.