Automatic product variations pricing I have a site where a client wishes to sell products that have a lot of different variations (ie size, colour, length etc), but each of these will make a slight change to the price. I know about the ability to make variations of products so that the price changes when you pick different items in the drop downs, but these have to be manually entered. For my client, each time the variations are open to the end user, the price increase will always be the same.

Both have the size option available to user -the price will increase on top of the base price by the same increment based on the selection. 

eg:
Small = no change
Medium = +£5
Large = +£10

Product A, starting price: £10
Product B, starting price: £20

So Product A with medium will be £15, Product B with large would be £30

I realise in this example it would be simple to add this in manually, but if she has 3 (or more) different options open to the end user, with upto 10 options within, multiplied by her entire product range, it would take forever to do.

What I had thought is if it was possible to add a price addition field to the product category listing, and then use a (rule/module?) to automate this price being added on. Is there anything already out there that does anything like this, or if not, how could I go about creating it.

Many thanks
How about the Commerce Customizable Products Module.  I just saw this video today, maybe it will help.

Drupal Commerce allows you to sell customizable products that require customer input when added to the cart. Use cases for this functionality include:

Business cards that require contact information and a logo upload.
Event registrations that require attendee information.
Donations the allow the customer to specify a donation amount.

The core Line Item module accommodates these scenarios by letting you add any number of fields to the Product line item type and expose them to the Add to Cart form. However, sites that sell multiple types of customizable products or a mixture of customizable and non-customizable products need to define multiple line item types to handle the different configurations of fields. That's where this Customizable Products module comes in!
It gives you the ability to define new line item types on the fly at Administration &gt; Store &gt; Configuration &gt; Line item types. This is functionally equivalent to defining the new line item type in code, which is still recommended if you are able.