i would expect around £50 per drive unit tweeters may cost less providing you can get away with just changing the diaphragm, But this is not an option on all models what do you mean by sounding odd?
I take it you have checked that the bridging bars / speaker cables are connected correctly?
I believe I'm right in saying that b&w dm610's date back to the early 90's and if you need to replace more than one drive unit your money might be better spent on an upgrade?
Something along the lines of b&w's dm686 at £279 or dm685's at £379 would be a vast improvement over the old 610's Loudspeakers have come a long way in the last 15-20 years!