Saying that 650 leva is a lot in smaller towns in Bulgaria is a myth, unless you live in a village, grow your own food and livestock to supplement your income this way. And keep in mind that the 650 leva figure is average gross, not net income of teachers. Thus you need to subtract 23% in taxes and social security payments from this amount.
Sure, 650 leva in a small town can buy more goods and services than the same amount in Sofia, but only marginally, not significantly more. Food in stores and utilities are priced relatively the same across Bulgaria.
Furthermore, the starting salary of a young teacher is 450 leva, not 650 leva, which is the average salary in schools. And again this is a gross, not a net figure, so minus 23%, you get 346.5 leva that a young teacher actually takes home after taxes. It is very hard to support yourself on 346.5 leva and it is close to impossible to support a family and kids on that amount. So a young person must either be extremely dedicated or extremely desperate to take up a teaching job. And there aren't that many people that extremely dedicated to pursuing a career in eduction. This is why the average age of teachers in Bulgarian schools is getting higher and the teaching quality is getting worse by the year, as the good and experienced teachers retire.
And if you think that the quality of textbooks in Bulgaria is great, you probably have not opened a textbook in the past 5-10 years. Most of the textbooks in Bulgaria are written by people, who have never taught in a school in their life, hence their knowledge of education is purely theoretical or from their own experience as students years ago.