Why not make it $10 on Amazon and $9.50 on all the other services. That way, people who are indifferent will go for the service that nets you the highest margin while people who prefer kindle won't care that much about the price differential.
Personally, I'm pretty much indifferent to iBooks vs Kindle since both are just apps on my iPad. If they're both $10, I choose Kindle simply because it's a bigger brand name. If there's even a tiny price differential, I'll choose the iBooks.
You can't. Amazon reserves the right to drop the asking price on your book to the lowest price listed anywhere else. ("Most favoured nation status" is the term used to describe this kind of clause in a contract.)
Personally, I'm pretty much indifferent to iBooks vs Kindle since both are just apps on my iPad. If they're both $10, I choose Kindle simply because it's a bigger brand name. If there's even a tiny price differential, I'll choose the iBooks.