People are Corporations Too
In many Western Countries, corporations are "people" under the law in a variety of situations; so, the best way to look at slave labor would be to invert this already questionable thought process, and treat people as corporations. A corporation can either be contracted to perform a service for you for a fee, or it can be purchased. In this since, a person who is hired is basically a corporation that you pay to do you a service, but a person you own is a corporation you've purchased and own. The reason why this is important is that we already have commonly used systems for determining the value of a corporation that could easily be applied to people too.
Since a slave's value comes from future earnings, and not current assets, the Revenue/Earnings method is the most logical approach to valuing a slave. By this method, a corporation is on average worth its next 5 years of predicted profit. In some really stable industries, it is more common to go off of 7-10 year investments, and so, perhaps younger slaves might qualify for a longer period worth, but even businesses that are super stable decade after decade generally aren't valued at more than 10 years of predicted income.
Like with corporations, this will be a subjective measurement. What skills and abilities your slave has to turn a profit with matters. Is he unskilled labor? a doctor? a pro athlete? Is your slave young and likely to become more productive as he ages, or is he old and unlikely to increase in value or worse, is he a high risk of suddenly going belly up? What is your cost of operation (food, cloths, etc)?
When you view human corporations like this, you can see their values could vary immensely. While an unhealthy unskilled Bulgarian factory line worker could be worth as little as €10,000, a young, successful football player from Luxembourg could easily go for over €10,000,000.
Why would people cost less than thier lifetime of labor?
Simply put, because slaves are investments. The whole point of buying a person instead of renting is that you take on the added risk, and reap the extra profit. If a slave (or corporation for that matter) costed as much as their entire lifetime of profit, there would be no incentive to buy one. In general, paying for something expensive upfront is not nearly as appealing as paying as you go, especially if you have no way of knowing exactly how long that thing will last or how long you will need it for. This is why corporations generally are not sold for more than 5 years of income, and why people would not be either.
How to motivate without violence
If the law says you can't murder, starve, or torture a slave, then you need to legally define murder, starvation, and torture. Which means you can stop just shy of that limit for an uncompliant slave. If starvation is defined as less than a 1400 calorie diet deficient in essential vitamins and minerals, then you can punish a slave with exactly 1400 calories of unseasoned beans and rice day in and day out, or you can reward them with a 2000+ calorie diet of seasoned and diverse meals. Not "abusing" them may not mean that you can't take away their mattress and bedding in the winter or turn off their AC in the summer, or blast loud music in their room in the middle of the night so they can't sleep, or withhold medicine when they are sick. Not harming a person is very different than taking care of them, and as long as they can work for you and only you, and you solely control the future quality of their life, then you have a lot of leverage to make them do as they are told.
In this since, modern slavery needs violence less than it did in the past because you could put a chip in a slave that employers need to scan for before they can legally hire a person. If you are chipped and someone scans you, then they are legally required to turn you back into your master just like a lost pet. This also means that an employer who fails to scan could face fines or jail time if a tagged slave ever shows up in his employment. Fully traceable tagging wont keep you from running, but it does mean that you won't have anywhere to run to.
How to avoid children getting involved
If you can't legally keep a slave who gives birth, then the value of women who can get pregnant goes WAY down. That is like buying a corporation that you know is likely to go bankrupt in the near future. This will generally lead to 2 possible outcomes. Women may become far cheaper than male slaves or women seeking to fetch the same price as a man will have to prove or provide medical sterility as part of their contracts.