So instead of us looking at this in an "OMG, Carey is playing well therefore he will leave in January" panic and instead envisage the scenario.
As Quintrell_Green notes a starting point for a fee would be wage x contract remaining. Given it is widely reported summer signings have option clauses for an extra year he has de facto 18 months left on his contract. I don't play football manager so I don't know the guesstimate wage they have. Lets go for £100k as a starting point (which would be around £1,250 p/w).
But we actually have a recent reference point in Conor Hourihane. He was our main source of goals/assists from midfield when he left for a reported £200k with 12 months left on his contract. So in Hourihane money Carey is worth £300k given the extra time on his contract.
Furthermore, the likelihood is Hourihane would have ran his contract down if he stayed so the club was in a much weaker position. We can keep Carey here 'til the Summer given we have the opportunity to sell him then; if he stops playing well the chances of anyone being interested then would decline.
Carey is three years older than Horihane was when he was sold, which would be a factor, but reality £300k is a conservative price tag.
So in terms of players in League Two who moved last January for money (see
here) I count three. All were younger than Carey is now, had at least 18 months first team experience in League Two and went to clubs in the Championship or Premier League. The two in the Championship got subsequently relegated and now play in League One.
So basically we need a team in the Championship relegation scrap willing to part with over a quarter of a million for a player with little potential to improve and has had 6 months experience of English football two levels below.
Oh, and Carey would need to think moving to a club with a high chance of being relegated to League One from a club with a high chance of being promoted to League One, and away from a Manager he clearly likes playing for, is a good career move.
Forgive me for not being the least bit worried.