We do at Capcom publish Street Fighter X Tekken and we did have DLC available via Xbox Live, however the service does appear to be winding down as you probably know, so the first phase of this is that it's no longer possible to buy from there.
Prices ruling at Steam today are £14.99 for the Additional 12 Characters Pack
Full DLC Pack and the Complete DLC Pack: £29.99
We're not explicitly recommending Steam as such - it's just a guide as to the price.
We are not yet sure how the situation regarding Games For Windows Live closing will arrange itself so I can only for the moment suggest keep checking on our own site, which is
www.capcom-unity.com. It is likely that news will be posted here as we learn more about exact times and we won't know more till then.