I fixed one of my toilets this past weekend. It wasn't completely broken; in fact, it performed its main function perfectly. You used it, you pushed down on the lever, and all the crap was gone. But apparently the seal between the bowl and the tank wasn't watertight and a little water leaked out -- nothing major, just the tank would refill every 15 minutes once the float sank about two inches.
As I repaired the tank, I thought about what that leak meant to me. It was only about a half gallon of water, so it didn't seem like much of an issue. Then I started to do the math: I was losing two gallons of water every hour. That was 48 gallons of water each day. Every day that I didn't repair it, another 48 gallons got flushed down the sewer. It still didn't seem like a lot. It was only 1,440 gallons a month.
It hadn't dawned on me that the water I was losing every 15 minutes meant money out of my pocket. (I get my water bill only once a quarter, so out of sight, out of mind). I also have a family that doesn't know the meaning of a short shower, so it didn't occur to me how much this could really cost. Then my sewer bill came: The utility devised a new rate based on the amount of water used. I was now paying for that leak in two separate places. Needless to say, my toilet no longer leaks.
My toilet episode reminded me of how many organizations handle BYOD. Their users choose their devices; if they are using them for work, in most cases they end up getting a stipend. In some cases that stipend covers the cost of the whole bill; in others, it covers a subset of the bill. As it's almost always a personally liable device (meaning your company didn't pay for it), the only discount you get on your contract is whatever your company may have negotiated for employees' personal devices -- and only if you remember to apply for that discount.
Now let's look at a large company that has invested in corporate-liable devices (those it buys and provisions to employees -- the old approach). Yes, they end up paying for the mobile devices the employee would typically cover under BYOD: $200 or so for most subsidized smartphones, and $500 to $1,000 for a tablet. But many large companies have the option to pool their voice minutes and their data bucket; if one person goes over his or her allotment in voice or data, the company can use unused voice or data from another person's allotment to make up the difference. That gets rid of much of the overage fees their employees would otherwise end up charging back to the company.