Amazon with Cambond has the Cambond Wireless Waterproof LED Door Bell (DB0201) for $15 - 5% off with Subscribe & Save (can cancel anytime) = $14 with free shipping. Sells for $26+ elsewhere. 

Comes with two receivers, has 500ft (150m) wireless range and 58 different ringtones with 4 volume levels from 25-85dB.