"Pushing 7 figures" sounds like it's probably around 900,000, which IMO is a fair valuation for Brannagan.
Matthew O'Riley from MK Dons went to Celtic for 1,500,000 this window and is a much better and younger player than Brannagan.
Only issue is that to Oxford, Brannagan is worth much more to them as they look to push for promotion.
They're kind of in a similar situation to us with Bowler (in that they would be willing to sell, but only for a very very high fee) except they will be even more desperate to keep him as they absolutely need him to fulfill their end of season goals while we don't really need Bowler to stay up.
Don't think this will get over the line unless we bid upwards of 1,500,000, which I think would be an overpay because he'll be cheaper in the Summer once Oxford fail to reach promotion.