I use(d) them for home games from MK, and I don't think it's Avanti that is the issue, I think it's more covid related, ie nobody used trains for months and months, and so they're trying to recoup the money they lost, they hadn't been doing it that long before covid, was it 12 moths ?
Avanti had good deals coming north pre covid, perhaps not quite as good as some of the virgin ones, and with virgin, if you had a season card you got an extra 25% off some trains.
I've not even looked this season so far, couldn't do Norwich, and didn't want to be saddled with a train full of that lot on Saturday either.