It's possible in Stora to create different prices for a unit type and set unique prices for individual customer subscriptions, here's how:
View your site, select the Unit Type tab and then press edit on the unit type on which you'd like to create a unique price
Change the price to the desired unique price and save. (This creates a permanent record of this price which you can choose later, but it also makes this the current price for any new bookings from the Storefront so the next step is important)
Go back into the Unit Type straight away and change the price back to what it should be for new bookings.
Go to the Subscription where you'd like to set the unique price and select "Add / Edit Storage Booked", you can then select the new price from the dropdown.