Here is the text leading up to the amount. The amount is listed right at the end.
The widow said she meant to give Huck a home under her roof and have him educated; and that when she could spare the money she would start him in business in a modest way. Tom’s chance was come. He said:
“Huck don’t need it. Huck’s rich.”Nothing but a heavy strain upon the good manners of the company kept back the due and proper complimentary laugh at this pleasant joke. But the silence was a little awkward. Tom broke it:
“Huck’s got money. Maybe you don’t believe it, but he’s got lots of it. Oh, you needn’t smile—I reckon I can show you. You just wait a minute.”
Tom ran out of doors. The company looked at each other with a perplexed interest—and inquiringly at Huck, who was tongue-tied.
“Sid, what ails Tom?” said Aunt Polly. “He—well, there ain’t ever any making of that boy out. I never—”
Tom entered, struggling with the weight of his sacks, and Aunt Polly did not finish her sentence. Tom poured the mass of yellow coin upon the table and said:
“There—what did I tell you? Half of it’s Huck’s and half of it’s mine!
”The spectacle took the general breath away. All gazed, nobody spoke for a moment. Then there was a unanimous call for an explanation. Tom said he could furnish it, and he did. The tale was long, but brimful of interest. There was scarcely an interruption from any one to break the charm of its flow. When he had finished, Mr. Jones said:
“I thought I had fixed up a little surprise for this occasion, but it don’t amount to anything now. This one makes it sing mighty small, I’m willing to allow.”
The money was counted. The sum amounted to a little over twelve thousand dollars. It was more than any one present had ever seen at one time before, though several persons were there who were worth considerably more than that in property.
When it was all counted, the treasure came to $12,000.
According to the first chapter of "The Adventures of Huckleberry Finn", the amount was $6000. Huck says,"Now the way that the book winds up is this: Tom and me found the money that the robbers hid in the cave, and it made us rich. We got six thousand dollars apiece—all gold." Judge Thatcher put he money was put in the bank for the boys and it gathered interest. According to Huck, "it fetched us a dollar a day apiece all the year round—more than a body could tell what to do with."
In "Tom Sawyer", it was 12,000 dollars. I believe they split the money since in "Huck Finn", Huck says he got 6,000 dollars.