Everything depends on the age and condition. If you are just doing straight scrap offer a few buck per tower. If they are older I would offer the same. Some of the older ones, the parts are a lot harder to sell. If they are more recent maybe offer a few bucks more because those parts may have a better resale value. I have never been able to resale a tower, cd, dvd or floppy drive so I pretty much scrap them. Hard drive with a large amount of space can be resold sometimes too. And you don't need skill to remove boards and drives. But if you are talking about getting them working, you go 50 attempts to get it right.