Even though a 500 MB scan may take 35 minutes, it does not tie-up the scanner, since you can be doing so many other tasks that need to be completed before putting your next drum in the scanner. There is no waiting around for the scanner to finish. So when you add it all up, it gets expensive Vs. The good news is, a good few year old used unit can be purchased very inexpensively, and these machines are tanks, and are fairly reliable. So if you need drum scans, and have a lot of film to scan, owning a drum scanner can pay for itself very quickly.
Labs seem to charge very high prices for drum scanning.
Do the math and figure when your payback would be. For me, it was less than 2 years. Drum scanners are large beasts. Mine takes up about a 2 ft x 4 ft area, plus the drum mounting station has about a 4 sq ft footprint. So you'd better have room to do all this.