What was the cost of a piano in the year of 1707?

$10 - $15 >>> todays money > 500 or 600$

Bail of hay and a keg of molasses.

Probably nothing, as the piano hadn't really been invented in 1707. Keyboard instruments were harpsichords, spinets etc. and, of course, the organ. Pianos didn't begin to become popular until the late 18th century.

Working and saving for 15 years to visit Christoforo in Italy in about 1722.