Magento Custom Option

I have a product on my store which is a wooden word. We offer a service that the customer can specify a ‘custom word’ Is there a way that the customer can select ‘custom word’ and be charged a base price - e.g. £10 They enter their word into a text field and be charged per letter - e.g. £2 per letter. So if their word was ‘Hello’, they would be charged £10 + 5 x £2 = £22 I hope that makes sense and I look forward to your help. and also implement this [login to view URL] into the cart options so it is in constant view when a user scrolls up and down the screen. I will need full explanation and sample code to implement in other sites