"Dear" is still used to mean "expensive" in BE and isn't what I'd call quaint, although it is more old-fashioned, and not as formal or business-like as "expensive". It isn't a complete no-no, though:
"The Trade Minister, Brian Wilson, welcomed the record surplus in services, but his Conservative opposite number, John Redwood, said the trade figures were 'dreadful' and placed the game on the Labour government's policies. 'They have made it too dear
to make things in Britain,' he said. 'They have driven sterling up too high and put up taxes too much. " http://www.guardian.co.uk/business/1999/feb/24/4
As a whole, though, the sentence "but we find your goods too dear" sounds odd to me for a business context. "But" sounds too crude (I'd go along with hm's "Unfortunately") and today contrasts oddly with "we find your (noun) too (adjective)", which is more old-fashioned/literary.