Suggestions for bridging two buildings only 30 ft apart

We need to connect two buildings about 30 ft apart, but we are unable to run a wire because of what comes inbetween (staircase and elevator). How would you guys suggest bridging the two buildings? we need to be able to get speeds of 50mbits with this bridge.

we looked into powerline ethernet, but from what i read, its too slow

we thought about wifi, but i am concerned about interferences like microwave (these are two apartment buildings), 2.4ghz cordless phones, maybe the elevator?.. and anything else. the distance is very small ..do you guys think wifi would work?

my goal is to get it done for under $75, even if we have to get used parts. any suggestions?

thanks