Hy depends on how big is your storage place, and if this space will be separated from where do you have your own merchandise store Don't forget if you have to move your merchandise its take time and employee to do that, it $$$$.

In reference to your prep space are they using your pots, pan, oven, electrical appliances, gas, etc. You need to take all these into consideration.

how many hours will they be there also, and how many are they? 

 I don't know your area but $15 or $20 per hour is not enough. 

Advice, may a contract and stipulate all your conditions.

good luck