a gil deal is a limited time offer with a huge pricetag, which may or may not return later on. the last gil deal we saw (i think) was 180 donuts for 3 playable characters, but there were also some buildings (like the bandit fort for 200 donuts without a character, or the golden statue from the last christmas event). you might be on the safe side with 200 donuts, but to be super sure you wont miss anything, stay at about 250-300 to have a bit of room in case something you really want drops
an example for a non-returning item sop far would be duff gardens, which was bugged for quite a lot of players, and has yet to reappear
