Your options are either hardwood (something like Iroko) or metal. Whatever you do, it is going to have to satisfy Building Regulations. Also it is almost certain that you will have to specify something that is custom-made. An organisation such as this one specialises in such stairparts.
http://www.stairplan.com/external_staircase.ht m
The price of the staircase won't probably kill you - it is the price of the work around it - foundations, fixing it, knocking walls out etc.
I wouldn't do this if I was planning on moving next year - simply not worth it.