That's an interesting observation. There's an ad for the 9/4/70 in the Timeline which lists the prices for that show as $4.25, $5.25, $6.25, and $7.25. There's also an ad for the 8/21/71 and 8/22/71 shows which lists those prices as $4.00, $5.00, $6.00, and $7.00. There's no ad with prices for the 6/25/72 show, but there is one for the 6/27/72 show. It says tickets were $4.50, $5.50, and $6.50. I checked The Concert File and The Press Reports and didn't see anything from '72 saying Zep was trying to hold ticket prices in check. Based on that, it seems like the '70 show was priced higher than normal. Maybe the sellout earlier that year of the March 27th show gave either Zep or the promoters the idea that they could raise the prices on the September show.