Flood sensors ($35 at Amazon) don't need bells and whistles to work well; they just need to tell you when your basement is flooding. You can make your own flood sensor and alarm for a couple bucks, but if you want one that sends you push notifications on your phone, one of the most affordable options is the $35 Aeon Labs Aeotec Water Sensor.
The big catch with some leak detectors, including Aeotec, is that you need a smart-home hub like SmartThings or Wink to use them. Why? Because the detectors function using a communication protocol called Z-Wave, which essentially allows them to work with low energy consumption, and send messages that the hub translates into alerts that it sends to your phone. If you want something that communicates directly with your phone without a hub, you'll have to dish out more money for devices like the or the .
If you do have one of these smart-home hubs, Aeotec is a great option for a flood sensor. It's simple, but it performs its function reliably. The only major issue it has is that it relies on a wired probe. Essentially, the dual-probe that most flood sensors use to detect water, is connected to a long wire, making it useful for attaching to the edge of a sump pump well or open floor drain. But if you want to just set it under a sink or in a particular problem area of your basement, its wired probe won't be ideal.
One problem every flood sensor needs to address is how it will be powered. Aeotec relies on batteries, so it will still send alerts even if the power goes out. The batteries should last well over a year, but you'll need to replace them to keep the device doing its job.
While Z-Wave and the wired probe end up limiting the Aeotec Sensor, they also mean it will work well for people with particular needs. So if you have a Z-Wave bridging smart hub, and want to monitor water levels in your sump pump, then the Aeotec Sensor is a great option.