Sold an attached lot to neighbor for a large gain from what it cost how would report on my 2024 return and fig. my basis. Would it only be what I actually paid for it?

Yes, the cost basis is what you paid for it. 

@Then2 there is a special rule for certain vacant land. I don't know if it would apply in your situation

gain from sale of vacant land that has been used as part of the principal residence can qualify for the home sale exclusion if the land sale occurs within two years before or after the sale of the dwelling unit. Reg 1-121-1(b). The land must be adjacent to the land containing the dwelling unit, and all other requirements of 121must be satisfied. the sale of the land and the sale of the dwelling unit are treated as one sale for the exclusion limit.