Easiest/cheapest way would be to rig up a sump pump to an overhead water storage container. Put a float switch in the storage container that triggers power on the sump pump and fills your storage container whenever it starts to get low.
From there just use gravity for all of your showers.
A decent 1/2hp sump pump can pump 3500+gallons/hour and are within the $100-$200 range and could easily keep up with 10 showers. Most shower heads now have a flow rate of 2.2 gallons/minute. 10 showers would be about 3000 gallons/hour.
A 1/2 horse pump can pump 3000 gallons/hour to a height of 15 feet.
For less showers and less pressure, get a really small sump pump, (1/6hp only pumps 1,000gph), like you would use for draining a hot tub. Then just run a hose to the shower. Wire a switch into the power for the pump and turn it on before you get into the shower. (A timer switch would work well to keep people from forgetting to turn it off).
For less than $100 you could rig up a single fully functional pressurized shower.