Thomas Jefferson, who died with $100,000 in personal debt, may not have been the most exact in terms of monetary estimation, although it still seems he may have intentionally downplayed the estimated expenditures when he went to Congress with his plan to send Lewis, Clark and a supporting cast to explore the West as his statement to Congress seems to indicate:
Their arms & accouterments, some instruments of observation, & light & cheap presents for the Indians would be all the apparatus they could carry, and with an expectation of a soldier's portion of land on their return would constitute the whole expense.
This proposal netted him approval of a $2500 expenditure to finance the adventure. One need only look at a fraction of the list of gifts taken to give to Native Americans to see why almost every dollar of this appropriation ($2324.00 according to National Geographic) was spent before the crew even hit the river; this list of items is somewhat lengthy, and as stated above, these are simply peacemaking gifts, not necessities, of which there were far more. To name just a few of these presents for Native Americans:
12 dozen pocket mirrors,4600 sewing needles,144 small scissors,10 pounds of sewing thread, silk ribbons, ivory combs, handkerchiefs,yards of bright colored cloth, 130 rolls of tobacco. . .
The final numbers when all was said and done was a total of close to $40,000. That number, of course, would translate to far more today, as, obviously would the initial $2500 appropriation. So, to summarize, the expedition cost so much because its predicted expenditures were, for whatever reason, drastically underestimated