Social security in Bulgaria
A foreigner must not pay social security in Bulgaria unless he becomes a local person with the center of his vital interests in Bulgaria, which means, to stay in Bulgaria more that 183 days per calendar year.
You may find English speaking skilled workers, who ask for salaries of about 350 - 400 euro per month.
The total social securities and the tax for the workers amount to about 50% of the net amount of the salary.
It is not possible to declare a less amount of salaries than the real one, because there is a penal responsibility for the employer for doing this.
There is also a minimum amount of salaries for which the social security payments are due for each combination of the type of the activity of the company and the type of the job of the employee.
Here is a template of a employment contract in Bulgaria.