For me, it varies wildly depending upon which type of bombing run I am making. I live in San Diego so the cost of transportation is minimal. Often I will drive to the border but I am also close to the trolley. I prefer escorts. The cost varies depending upon which agency and how long you want to monger. The costs for some of the nicer hotels (Premier, La Mansion, La Espaٌa, Haciendo del Santiago) have actually gone down. The actual costs can range from less than $100 to over $300 or more.
For me, the cost of BGs is usually less than the average monger because I loathe spending time in bars. The music is ear-splitting and the smoke is bothersome. I'm trying not to sound elitist but I find the behavior of some of our fellow gringo mongers downright embarrassing. I usually make a few trips scouting about until I find a suitable chica, then quickly make my move. One ficha and we go arriba. Usually less than $100, sometimes much less, sometimes more.
I haven't done a SG in a long time but that cost can be almost negligible. The peso has fallen so 200 pesos is currently around $16 and the cost of those hotels which shall remain nameless is often less than $5. Try to find a deal like that in the States!
Sorry that these numbers are not too specific, but as I said, it varies wildly.
Yours in mongering,
gmbrldx