Not too much! Truthfully, we only get a small "per diem" for our time on the International Space Station (ISS). I believe my bank account had a deposit for about $172.00 US after my 152 days in space! That comes out to about $1.20/day! But when you consider that we have room and board accounted for (provided), and we are not going out for dinner and drinks every night, it's probably a reasonable (but cheap!) government estimate. Our MI&E (meals, incidentals and expenses) don't amount to much while we are sailing around the Earth at 17, 500 mph! Overall, we receive our normal annual salary whether we fly in space or not, and -as you can clearly see- we don't get much extra when we do fly.
I recall while on ISS talking this subject with my Russian crew mates. While not divulging too many details, I learned that their annual salary was about $6000 US. For their 6-month stint on board the ISS, they would make 10 times that salary!Now you know why they are so interested in flying multiple times. It can be a big advantage to their pocket books! In addition, if they performed tasks that weren't on the originally developed timeline/manifest (a manual Progress docking for example), they would also receive additional funds. I speculated that this was another reason why they were very cordial with the ground, no matter how upset they were in orbit... they didn't want to upset anyone and jeopardize their pay. Too bad the US didn't do it that way... I might have kept my mouth shut a bit more!
A recent Robert Frost/NASA JSC answer on Quora gave more exacting details on salaries for US civilian/military astronauts and Russian cosmonauts. I suggest you check that answer as well.