Multi-channel retailing: research about offline driving online sales and vice versa

So how do we go about comparing ROI of off line and on line marketing activities.

I have been calculating a ROI figure for PPC activities. This is calculated based on the cost of clicks one month, and the gross profit on sales from those clicks and sales from those leads during the next 3 months.

My next step was to do a similar thing for off-line advertising activities.

As your post explains cross channel behaviour is quite a big factor, but how would you suggest modifying these calculation.

We are the UK's leading supplier of dolls houses & miniatures, our primary market is UK mail order, on and off-line, we also have a couple of retail outlets and also have a B2B sales channel.

Any advise would be appreciated.