mod_perl is typically 20-40x faster than vanilla CGI. That correlates to 20-40x the capacity or headroom in rough terms. So all other things being equal you can run vanilla CGI and buy 20-40 times as much hardware or you can go mod_perl and generate the same throughput. This is a no brainer.

As you correctly note there is average load and peak. With no change in hardware you have 20x + the headroom with mod_perl to handle those peak loads which is often the issue. A handful of hundreds of request per second is the mod_perl ballpark. A handful or several of requests per second is the vanilla CGI ballpark.

PHB: I've heard that with mod_perl we can handle 20x as much load with
+ the same hardware. 
PHB: Is that true?
You: Well yes but our code is badly written and mod_perl is um kinda u
+m new and harder....
PHB: So you know that we are in this to make money and hardware is a f
+ixed cost?
You: Well yes but...
PHB: So if you make it work with mod_perl we can save $XXX per month o
+r.....
You: Well yes but...
PHB: What we have here is a *failure* to communicate.....
[download]

30000-50000 hits a day is fairly trivial. That is less than 1 hit per second on average although the peak may be up at 10-20? Parse the logs if you don't know. At these peaks vanilla CGI will aproach its limits. mod_perl will hardly raise a light sweat. Maybe it will never make any differnce. Maybe you will become really popular. Maybe you will crater because you could not handle the load? Dunno. The business case is simple enough. mod_perl = more reqs/sec for the same hardware capital cost.