We rent out a number of machines on a yearly basis but invoiced on a fixed amount monthly. The agreement calls for the machines to be filled and refilled with certain wasting liquids over the contract period. We also sell the same machines to the general public. But we have a different fixed prices for the machines on rent because some other maintenance costs are involved over the period. 

My questions are:

1. Do we have to keep 2 separate prices for direct sales and rent for the same product. Is it feasible to do so when the machines are issued from the same warehouse.

2. How can we tract the profitability of each contract? Can we use project application or can it be tracked via the subscription module? For example we would like the cost of material issued and other cost relating to a contract be assigned to that contract. 

3. I need the sales order and invoices relating to any contract to indicate the quantities and rental prices per devices. Is it better to use the rental application or the subscription Application? I use Odoo 15.