Basis re-creation, as best you can, and any estimation on the low side. Document what you do & any assumptions made. Weirdest one I had was an elderly man who had been investing in a mutual fund through payroll deduction, for years. He knew it was $5/week, started about when his son (older than I am!) started kindergarten. How long? Well, he left that job, and we figured out how many years at $5/week. I documented everything. It was still a pittance compared to the total sale, but it saved him some tax, he felt better about the whole thing, and I had copious notes to back up the assumptions made.
Charge for the time. (I didn't, but then my client was in his mid-90's and it was worth it to hear the tales of his work and his son's scout troop shenanigans and the rest.)