Oh boy, get ready for the responses. You'll hear everything from "$0 tip as I've already paid the guide for a fishing trip" to "50% is standard".
First, I'd pray for rain as they've reduced the release from Cannonsville again and the East is all but dry. Without water, floating would be difficult at best.
I'll echo some of what Tom has already touched on. If you aren't familiar with the fishery, it has the reputation as a notoriously difficult place to catch fish. With the low water up there this year, the fish have been crowded into thermal refuges and relentlessly pounded by fishermen. If temps cool before you head up and the fish spread out again, the odds are they will still be skittish / guy shy.
Without a tropical storm dumping rain on us, I'd expect you to find low / clear water conditions. There are insect hatches in the fall but they are not nearly as heavy as the spring hatches. The hatches tend to be spotty and shorter in duration. You will also find the bugs to be on the smaller side when compared to early season hatches.
On the plus side, after Labor Day can yield pleasant temps for fishing, stunning colors on the mountains and short spurts of fish frantically feeding. Success on this river system is measured differently than on a typical PA trout stream. A good day brookie fishing might be 50 fish, a good day on spring might be 30 fish and a good day on the Delaware 'could' be 1-2 fish. A lot depends on the conditions and angler ability.
Getting back to your question, there is no standard tip. I have a buddy that guided up there and he's gotten tips ranging from $0 to $150. As Tom has mentioned, gauge your tip on the enjoyment / fun level of the trip and not solely on the number of fish in the net. You may be met with brutally tough conditions, catch two fish and then head to Lydia's for a ribeye sandwich. You end up talking to 20 other guys that were on the river and they didn't even sniff a fish. So, your 2 fish beat the total for 20 other anglers. I'd call that a good day. Gauge the tip on the effort put forth by the guide and if they made the trip a fun experience.
3 MUSTS for your trip...... camera, rain jacket and fun.