When did the practice of using monuments like the Coliseum as marble quarries for other buildings end and why? Was it mostly a factor of changing costs or did the respect for these ruins drive the change?
I think it was a combination of the two factors you mention:
The rise of the "Grand Tour" in the 17th century (the beginning of modern tourism) and the beginnings of the formal study of classics in the 18th century meant that old ruins were increasingly more valuable intact as cultural objects than as a source of raw materials.
In the 19th century the changing economics of transportation and quarrying (mostly due to the introduction of steam engines) meant that it was no longer cost-effective to try to mine ancient ruins directly for building materials, especially since many cities in Italy and Germany were increasingly larger than they had ever been before, making and use of new sources of building materials a necessity anyway.
The Getty Museum in Los Angeles put out a decent coffee-table book Irresistible decay: ruins reclaimed back in 1997 that discusses the changing cultural perceptions of ancient ruins. There's an important point here is that there is a very particular cultural framework required to view the remains of old buildings as "ruins" which are imbued with cultural and historical meaning, as opposed to just old piles of rocks.