Thanks for the reply.
We are/were using the Summary cards with large images. We generate a unique HTML page per card which is pretty simple and easy to do for us as we control all the workflow. Since the HTML page is small its easy to create a new and unique one for Twitter Card we wish to create. The meta tags are created as part of this page.
Our understanding is the Summary Card with Large Image is free.
We understand the issues with lack of 3rd party support for Twiiter Cards, if Twitter wants to keep this technology to differentiate itself from 3rd party clients, however our view is that there needs to be a way for 3rd clients to ‘gracefully fail’ from Twitter Cards through to more basic Tweets. You mention attaching an image to the Twitter Tweet which we have already tried. The image is displayed but we end up with just the image and a URL in the Twitter IOS client and on the Web. It seems to us that the act of putting a media image and/or text in the tweet along with the URL to the web page containing the correct Meta tags, stops Twitter Cards working at all and we never see the Twitter Card. If this is wrong and it should work please let us know as we explicitly built and tested code for this type of upload.
We seem to have a very inconsistent and I have to say poor user experience amongst the different ‘official’ Twitter clients on Mac, Web, IOS and Android. The IOS Twitter Card displays very little text in comparison to a simple Tweet with an attached image, the Mac client requires you to click on the Tweet in the timeline to get it to expand (the less said about the Mac client the better), TweetDeck on the Mac seems to struggle to display the Twitter Card at all. The web interface requires us to click on the Tweet to get the Twitter Card (this is so annoying).
We would love to use a Twitter Card that is consistent across all the clients. However all the clients we use, appear to be at very different stages of maturity and have different behaviour. We understand that clients development moves on at different paces (we are developers ourselves) but we need consistency OR the ability for Tweets to fall back displaying different levels of information depending on the client, e.g. if Twitter Cards cannot be displayed then fallback to a Tweet and an Image, then fall back to Tweet and a URL and then fall back to a URL.
We cannot see any way to do this and since a proportion of our clients use 3rd party clients (nothing I can do about that) we have to work to the common denominator that provides a reasonable user experience which is not to use the Twitter Cards but to send a Tweet with an Image which works across all clients but has the downside of requiring people to do more clicks to get to our website which means we lose traffic. We love the idea of a single click on a Twitter Card going straight to the web page and would be happy to pay an appropriate amount for that functionality across ALL (1st party and 3rd party) clients. Thats the way we want Twitter to work for us.
Since we cannot have Twitter Cards across enough clients (including Twitters own) AND we have no fallback method (that we know about) that works consistently across these clients then we cannot see any other option than to stay with the current Tweets which work but aren’t driving the traffic we would like.
Apologies if I have misrepresented anything here and what we need to do is obvious (if not to us), the team has burnt the best part of a week exploring options, writing code uploading its testing it, and we believe we have tried most options. We have read most of the forum posts “just-in-case” somebody had mentioned a solution but we can’t see how to use Twitter Cards and provide a rich and consistent user interface to our clients.
I’d be interested in your views and if we have missed a simple and obvious solution, would be delighted to be proven wholly wrong 
Best wishes,
Rob