Under $1000 at homedepot. Look in the plumbing department and they have the information to show the payback period.

i put in a rinnai tankless...it's great; around 2k installed. gas, endless hot water, adjustable thermostats at as many locations as you'd like (ie when in the shower turn it to 110 degrees, when doing laundry, 140) etc. The only downside is descaling it every 6 months which takes about 15 minutes and 3 gallons of vinegar (very hard water in western mass). Saves a lot on the gas bill; and we left the old hot water heater in line before the new one, so water is "preheated" to room temp by sitting in the basement, as opposed to coming out of the well at colder temps (also prevents some of the scaling).

Rinnai makes a lot of excellent gas systems, including the best space heaters you will ever find - quiet and efficient, with a humidifiying system.

My brother installed one in his home a few years back and he loves it. He did admit that he should have gotten two smaller ones at opposite ends of the house. His is big enough to handle a shower and a washing machine at the same time. He told me he'll eventually plumb one to the washer and dishwasher and another to the rest of the house, both smaller units of course.

EEMAX Electric Instantaneous Water Heaters
Hot water on demand. This unit turns on automatically and shuts off again when water flow stops – no waiting for a storage tank to heat up. Requires only one cold water input line and mounts on bulkhead.

Constructed of high strength reinforced salt water resistant plastic and stainless steel.

Runs on 120V shore power
Compact size (10.75”L x 5.25”W x 2.125” D)
Amps 2.9
$269.00
http://www.westmarine.com/webapp/wcs/stores/servlet/ProductDisplay?storeId=10001&langId=-1&catalogId=10001&classNum=28&subdeptNum=27&storeN um=11&productId=56281

I have installed about 10 of them, I use TAKAGI, they have excellent customer support. If you do not use stainless steel exhaust tubing, condensation can occur which creates an acid which can eat thru the copper heat exchanger after time. The tubing is expensive, and if it is going to be inspected, probably wont pass if inspector asks to see spec sheet if not stainless.
Most units require a controller so you can change water temp at will.
controller is wired in and can be as far 50ft from unit.

not AFFLIATED with TAKAGI, just went to factory training for installs