We'll be leaving for Phnom Penh for 1 day, would like to know if there are storage lockers near HCMC and how much will be it's rate.
There are definitely luggage lockers at Ho Chi Minh airport, although this might not be ideal for you. Unfortunately the internet says that there are no luggage lockers in the train station. Your best bet is likely to be asking your accomodation to store the luggage for you. They might do this for free if you promise them you will come back for another night after your trip. Others might charge you a small nominal fee for the service.
In general, don't leave valuables in your left luggage. There is no guarantee that the luggage will be locked away, and establishments usually deny all responsibility for damage and theft.