Do you have a model #? I thought those magnum energy inverters where big $$$$. I did see one Xantrex unit - the Prosine 2.0 - that has a 2kw inverter, charger (100A) and auto transfer circuits, but even that was over $1200... and way overkill for small backup needs. Probably need over 500Ah of battery to feed that beast.
The cheapest I found was this....
http://www.westmarine.com/webapp/wc...gclid=CM_Sr-r3srMCFUWo4AodKRQAzQ#.UJUgZYXP8uw
but I don't really need auto-transfer...let alone to pay that much...just blue skying different ideas.