Is renting an option. We rented a huge waterslide for DS's last birthday and it was about $350 and included delivery, set up, and haul away. The generator was about $25 extra. It was so much fun and the adults had just as much fun as the little ones.
We were tempted to buy a Costco slide too, until we saw the weight limits and recommendations. I think that only 2-3 kids could be on it at a time or something. I know with the big water slide a lot of the kids like to just hang out at the top and get wet.
DS 10, 5th grade
DD 8, 3rd grade