Pro-rata prices

[sgill24seven]

Hi I'm using Magento ver. 1.9.2.1 CE

I'm making subscription based products which will involve taking a month payment in advance plus pro-rata cost of current month and then taking payment by direct debit for the rest of the subscription.

Firstly I have set up an attribute set called contracts then i have assigned attributes to it such as requires direct debit and contract length.

What would be the best way to add the pro-rata part to the product? I was thinking it could be connected to contract attribute set or the requires direct debit attribute. Or even simpler maybe add it as a part of a bundled product but how can you set the pricing of that part of the bundle to be a price pro-rata based on the day of the current month?

Thanks 

[Brent W Peterson]

Are you accomplishing this using default Magento or do you have a module that is doing your subscriptions?

[sgill24seven]

Thanks for your reply. No this is default Magento. I'm basically trying to make a subscription without modifying too much, i think the best way would be to have a bundled product consisting of the contract/subscription and then another product which will be the pro-rata element. 

What would be the best way to edit pricing functionality. For example having a tick box in the pricing section of a virtual product that says pro-rata, and then when this is ticked it calculates the pro-rata price that day based on the initial price/number of days in the month*days of the month remaining.

Example

Lets say it was £10 per month so you set that price as normal then tick the pro-rata box
If we sign up today and start subscription tomorrow it would be £10 / 30days in november * 25days of month remaining = £8.33

Hope that is clear, Thanks.