it depends on how common the size is... i was looking at 17" tyres for my 17x9 +15's on my R31, and due to being quite an aggressive offset, i preferably needed something under a 235.
a 235/45R17 is much cheaper than a 225/45R17, which is cheaper than a 215/45R17.
i went with 235's all round, and if i do get a bit of rubbing (it's barely touching, but it does happen), the tyres are alot cheaper to replace!
in regards to the original post... have a look at different size/profiles around 235-265, you might find a common size that is a bit cheaper.
235/40R18 is actually a 'legal' fitment on a 18x9.5" rim, and doesn't look too stretched.
edit: this is a 235/40R18 on an 18x9.5 on a mates 33 gts-t.