If it's grade 2, you'll quite likely have to replace windows and doors on that converted bit, they stand out like a sore thumb in the photo, which is a whole lot of money (can't remember what ours were, woman before us did them, but it was in the thousands for 3 windows, and more again for the 2 big doors).
And I bet they didn't maintain the roof if the rest of it looks like that - that'd be much money to do - ours being in a world heritage site, last owner managed to get a grant to do it, but it was still an awful lot of money.
It looks pretty shonky, the barn bit in particular. If you had to redo much of the extension, do the roof, windows and doors, sort out the walls, that could cost a mass of money. Listed stuff is not cheap to do - for example the rebuild cost on our house is estimated at something like £500,000, for a house costing less than half that money, and that is for something probably not massively bigger than your great big barn.
Oh, and the reason it is cheap is probably because it is massive and shonky. Selling massive old houses even in good condition is silly hard and takes forever. Be aware of that, if circumstances change and you need to move expect a long time to wait before it sells.