You can get a battery back or a water driven pump for between 150 and 400 bucks. Still have to worry about battery running down and replacing. Get an insurance rider on your policy to cover sump pump backup. Most don't cover it on regular policy.
Do you have a sewage uplift pump. If so add a floor drain to it close to the sump pump, then it can catch the overflow. I recently spent part of a power outage during a storm bailing my sump pump with a bucket. Not much fun. I was wishing I could find that cheap drill pump that hooks to a water hose, but I've got more tools than I can find when I need them.
I just happened to walk down into the basement or it would have overflowed and I would have missed it. Go to the plumbing supply link for a choice of pumps. They have the whole setup. http://www.plumbingproducts.com/12volback.html Try this alarm for $26 http://www.plumbingproducts.com/alarm.html Also check out www.plumbingsupply.com