From what I see, they are cheap. Standard PVC electric valve activated by the float switch. A glorified larger potty linear float activates a mechanical sealed switch.

One type you simply tie the wire onto a post in such a way that when full, the float is wire-down, and low, wire-up.

http://www.essentialhardware.com/product_detail.php?pid=226451&gclid=CJyP6o_5t7sCFew7MgoddTEAoQ Cheap enough!