> The exact amount they can get away with depends on personal tolerance and your time value for money, but given how much moving sucks, in practice it's quite a bit.
It is my experience, as you say, that rent tends to increase by a small amount each year, usually about 2%. Even people on fixed incomes tend to get cost of living adjustments, though, and the range is about the same as typical rent increases.
I do not see rents increasing by the amounts you suggest, which suggests that either all landlords since I started renting in 1991 have been leaving money on the table, or they can't actually do this reliably.
I think there are several reasons why they can't raise rents by 10% a year in order to keep people there, even though that extra money would be about what it costs to move:
Since the money it would cost to move grows only at inflation, by the second or third year it would suddenly be profitable to move, even for renters who really hated moving. In order to avoid this, landlords would have to apply the rise only once. But if they do this, then the market rate is just the higher one, and you could look at the initial lower rate as a promotion. In fact, this is quite common in rental advertising! Often it comes in the form of "first month free", which is about of the same scale.
More importantly, people who rent probably do not expect to stay in the same house or apartment for many years. In non-rent-controlled areas, rent is more expensive per month than buying the same sort of place, so if you intend to stay in the same location for more than a few years, there's no reason not to buy. As a renter, I've never lived in the same dwelling for more than three years, and when I move somewhere, I assume that I'll be moving again in a year or two, as conditions change (job, relationships, etc).