You can get even cheaper water injection on a supercharger set up. Using the manifold pressure available you can pressurise the water reservoir to squirt water into the SC inlet. Since it's a roots blower it'll actually improve it's efficiency by acting as a tip and side sealer. (Not sure how much though.) All you need is a water reservoir that can handle the boost and is airtight, a nozzle and maybe a restrictor (valve/ tap whatever you want to call it) to get the water flow right. Atomisation isn't as important in this application, just getting it distributed along the blower to some degree. So, if you used second hand or discarded parts like hose and a soft drink bottle you could about do water injection for free.