From my experience in a UFCW union at a supermarket PT employees probably start at $0.25-0.35 above minimum wage and get a $0.25-0.35/hr raise each year, depending on which dept they work in. Ironically, a cashier would get more than the people working the Deli, Bakery, Produce, etc. Apparently scanning bar codes required more skills.
Of course subtract $7-9/wk in dues, which WILL increase right after you union negotiated raise.