The cost is $299 for the regular ResortLock and $399 for the more weatherproof version.  If your door is under a porch or some other way not directly exposed to the rain then you can do the $299 version.  You can preset the lock with the 4 digit codes you like for yourself, housekeeper, etc.  Then you visit the resortlock website from your phone or your house where you live now and generate a code to give the renters that is based on their arrival and departure times.  You can generate them for up to a year in advance I think.  I usually just generate mine the week of the arrival.  After their departure, the code no longer works.  The website sends the renter an email for you with instructions on how to enter the code and how to generate their own 3 digit code that will be valid during their stay.  The ResortLock has a key as well, you can put that in a lockbox or hidden location if you like but I'd definitely hide it so you don't have vandals/criminals trying to break open the lockbox to obtain a key.  You replace your original lock so any keys you have would work with other locks on the house if you chose to.

Rick

www.vacationrentalhelper.com

If you consider that having a lockbox and a physical key is not much different than having a keycode that goes unchanged between renters, then simply switching over to a keyless lock.  In fact, for all intents and purposes, that's what your current lock box must be - it's just that instead of typing the code on the door, they enter it in the lock box, then use a key to open the door.  So having a key-code door for your guests and cleaner is no step backward (nor forward).

Sure, there are units that can be changed remotely between each guest (via internet).  But changing the code manually really isn't so hard that a housekeeper can't do it - ours just requires typing in a 6 digit master code, then entering the new 4-digit code 2 times in a row.  Or to reset it and wipe out all but the pre-programmed codes, they type in the master code followed by 2 more keys and it is all reset to factory settings.  It's really very easy.  And it also has a real key/tumbler so you can get in even if all the codes are lost or the battery is completely dead.

As for electricity, nearly all are battery powered, and for us, the battery lasts about a year of normal weekly use.

We have the Schalge non-internet locks on our houses <http://consumer.schlage.com/Products/Pages/category-landing.aspx?category=Electronic+Keypad+Locks>  We like them because they are easy to program, they have a real-key over-ride, and the electronics only release the lock - the deadbolt is actually turned by the person at the door.  This design uses much less electricity than the type that also move the deadbolt - so the battery lasts much longer.

There are many very expensive, internet controled systems that track people coming in ang going out, but for simply lock-box replacement, a $90 Schlage deadbolt set is the best answer.