One, but not the only.
http://www.history.com/this-day-in-history/suez-canal-opens
Thirteen years from the formation of the company, ten years from starting construction, and four years behind schedule - despite using 19th century technology (as opposed to what would be available earlier) for at least a good part of the work.
The misapprehension about sealevels dates to a hurried survey during napoleons occupation. Afaik, that was the first modern survey .... and the only one to get it wrong.
Ya, de lesseps original methods werent far from what was available a thousand years before ... and were grossly inadequate for a commercial company.
Note, too, that there was a major ridge of rock in the way near the south end that was relatively easy to deal with with tonnes of gunpowder, but would have been much harder with pick and shovel.
Otoh, they dont need to make it as deep or wide, so thats easier.
Ot3h, they may need tow ways, which would make construction interseting, especially in the lake sections.
Moreover, maintaining and dredging that size canal by hand would be ... fun.
Id say 25 or more years to build, and a sizable expense to maintain, which will reduce profits, and extend payoff time.
Remember, too, that all that capital investment is up front, and itlll bbe a generation before ANY money comes in. At 3% interest, that would double the cost. At 100% it increases the cost millions of times.
Real interext rates would be inbetween.