I think it was something around 12 mil, if I remember correctly.
You basically unlock the option to sell your town once you are able to sit at the plaza tree. If you go to the
game's menu and go to the part where you can delete your current town, Isabelle ask you if you want to
start a new town after you deleted your current one. Once you say yes, Nook shows up and makes you
an offer for your current town. You have the option to also sell your catalog, however, if you keep it, you
will get access to it with the new character you create in the new town. The bells you will get by selling
your town to Nook will transfer to the bank account from your new character. So yeah, you don't keep
the same character and you only keep the catalog (if you don't sell it), the bells you get for selling your
town as well as all PWPs you unlocked in your old town (but only if you sell your old town as your mayor,
like by starting the game, you have to choose the account of your mayor. Also, only the catalog from one
character will transfer to the new town).