how do I extend the range of a summer baby video monitor

I don't think this is going to be possible (or at least cost effective).  If the monitor receiver had RCA jacks, you might be able to forward the signal on with a wireless A/V sender like this one:  http://www.radioshack.com/product/index.jsp?productId=2799800.

However, it did not look like any of the Summer models had RCA out jacks.  The only other way I could think of would be to get a device that would extend/relay the 900mHz signal the baby monitor sends out.  I have never heard of such a thing but if you do a google search for '900mhz repeater' or '900mhz signal boost' or '900mhz amplifier' there are a bunch of sites out there.  You might try contacting one of those companies and tell them your situation.  Also, you might try contacting Summer and see if they have a suggestion.  

You could always make your own by buying a wireless camera or a regular camera and use the radio shack-style wireless video senders.  A webcam might work too if you have internet in both locations.

Good luck!

There are phones that will do 'sound' room monitoring up to that distance, but you'll never find a video baby monitor to do it and it would be far more costly to figure it out vs. a more professional solution.  I'd actually look at a web camera setup in the room that is somehow connected to your PC and can then send through a wireless router to your location 500ft away.  I'd say that the best solution would depend on what you already have available to you.

Just run cable along the fance, or else............With RG59 you can go up to 1000",  and roll of 1000" you can get on ebay around $60.
Heck with the wireless- it always worst case scenario to deal with- either changing battery, have outlet nearby, have interfirence, loss of signal and so on- endless headache.