Run the contact output from the sensors and wire them to the terminals of the HAI wireless transmitter.
That's about as cost effective as you are going to get.
The $10 solution is just a battery, a couple of resistors, a transistor, a piezo sounder and some copper contacts.
Velleman sells one as one of their mini kits for a few dollars. Look at the schematic to see how simple it is.
When you start adding wireless RF communications, communications protocols between the processor and transmeiiter/receiver, etc., you are adding costs.
Since you allready have the receiver, you have the most expensive part.
The 3 zone transmitter I posted can be had for $30 or so. That's around $10 a zone.
Add some GRI 2600 contacts for ~$12 each, and a $5 12VDC wallwart to power them, and you are very cost effective and a much better system than the $10 sounder.
Since HAI doesn't make a ready made wireless water sensor, you have to do a little work
But with just a little effort I think you can get where you want to be.