Without the overprint, it is a Russian semi-postal stamp Scott #B13. I don't know what the overprint makes it. Maybe a revenue or other Cinderella or local stamp.
With a different surcharge and overprint, this stamp was used in Armenia and Siberia, but not this one.
This is one of the stamp trading revenue stamps issued in 1925. According to the Michel catalogue its value is 25 euros. If yours is perf 12 1/2 it is 100 euros. I do not think Scott lists these (almost certain of it)
I have never fully understood the reasoning behind these special revenue stamps. Stamp collecting was considered bourgeois, yet if you wanted to exchange stamps abroad, the government thought it was fine, as long as you paid a special tax. Weird.
Interestingly, it is my understanding that when sending stamps to a collector in Bulgaria, you have to send the stamps to the state philatelic agency, which follows checks the stamps to make sure they are permitted into the country, and then forwards the stamps onto the collector. I'm not sure if any taxes are assessed, but I wouldn't be surprised if they were. I found this information in the International Mail Manual from the US Post Office.
I have received stamps from a dealer in Bulgaria. There was no indication on the envelope that it went through the Bulgarian philatelic agency.
