Well I don't have anything to do with AMG, but did a fair amount of business with them quite a while ago (over 10 years). I also ran one of their largest competitors back in the day.
I can tell you that the creators of AMG are first and foremost music lovers, who are in the business more for the access to great music that they enjoy, than to make a buck. That being said, they do have a right to be paid for their hard work, and they have gone through one or more corporate owners in the past several years, that are certainly more financially focused. (last I heard, they were owned by Alliance Entertainment, a large music distributor).
This plug-in did make their data less valuable to their corporate clients. Why would an on-line music retailer pay AMG $1000's to license this data, if the stores customers can already get access to all the data for free.
However, I'd say that your plug-in DOES make their data more valuable, in that it gives end-users access to their data in a way that's usefull to the end-users. In-short, AMG or one of it's business partners should buy your plug-in and sell it with a reasonably priced data subscription.
I presume you've already replied to them and told them that you don't intend to infringe on their copyrights. The lawyer wrote you a pretty nice C&D letter, you might consider contacting him again and asking if he could put you in touch with a Biz Dev person at AMG. See if they might have any interest in paying you for the plug-in, rather than you paying them for the data.
Just my $0.02.