Okay. Say you buy 30% of the wards smoke etc and your 5 buys 70% of it. 150g observers every 6 minutes, 200g on sentries every "6" minutes, 100g on smoke every 5 minutes and say 180g on dust every 10minutes (more or less, don't know what enemy heroes are).
So in a 30 minute game,
150 * 5 = 750
200 * 5 = 1000
100 * 5 (you can't really smoke more than 5 times in 30 minutes unless you manage them extremely well) = 500
180 * 3 = 540
Total: 2790
so your 30% is 837. 837g is really not a big deal over a 30 minute game...