Whirly; I would agree that an SEP is about 2.5 times more expensive, but realistically you need the MEP hours to get noticed in the job market. I believe BFC's twin was about the same price as an R22 - so it could change the figures quite dramatically. That's why I would really recommend to draw up a budget first.
4KBeta; At the moment following the 'interim' procedure set out by the JAA, there are 12 exams for ATPL level that are common to both (A) and (H). In addition to this for aeroplane you must also sit POF(A) and performance. For helicopter only POF(H). That makes 14 aeroplane and 13 helicopter. Clear as mud!