|No such thing as a small change|
I guess that depends on how much revenue the ad produces, and how much memory your app consumes.
Dotcloud's pricing is based on how many horizontally scaled instances you need, and how much vertical memory scaling you need.
The default configuration with Dotcloud is for there to be four processes running. So your 16MB app will consume 64MB. A single 64MB Perl service costs $8.64/month. The smallest vertical scale you can select is 32MB, so your application would have to consume no more than 8MB per process. There may be a way to configure fewer processes, but I haven't looked for it.
Now for horizontal scaling: If you want your service running in a couple of instances on multiple availability zones, you can specify to horizontally scale to two instances. Now the $8.64 doubles to $17.28.
The main reason for scaling vertically is to obtain more memory reserved for your application. As you scale vertically you also get more storage space, but that seems mostly an issue for database services.
The main reason for scaling horizontally is reliability. Dotcloud automatically places each instance in a different availability zone with Amazon EC2. Dotcloud also handles automatic failover and load balancing. For database services that are horizontally scaled, you get automatic database replication as well as failover.
The Perl Regex Tester is somewhat memory hungry. It uses Moo, Safe, Try::Tiny, Capture::Tiny, Sys::SigAction, Time::HiRes, and Carp within its model class. And in its application class it uses Mojolicious::Lite, which itself pulls in a lot of the greater Mojolicious framework. Plack is also in the stack. And then there's the big unknown: What creepy regex will people throw at it?
I've enacted timeouts to limit the damage of a grossly inefficient regex, and also limited the amount of output from "use re 'debug'" that I capture. Both of those constraints reduce the potential for time DOS as well as memory consumption. I also limit the size in bytes of the regex and of the target string thrown at it. But still, I've seen the four processes jump to a total memory usage of around 256MB. I could probably clamp down a bit more on the timeouts to ensure that it stays below 224MB (remember, that's divided among four workers). A 224MB service that is not horizontally scaled would cost me $30.24/month. However, since I don't care about redundancy, and I don't need a custom domain name, and I don't really care about performance guarantees for this particular app, I just run it as a free sandbox application. If someday someone decides they want to support converting it to a live (paying) app, I could do that quickly, and I'm sure Dotcloud would be happy to accept payment. ;) By way of comparison, when I first start the application and throw a bunch of simple regular expressions at it, the memory footprint stays between 64MB and 96MB as a total for the four workers. In other words, if I had stricter enforcement of complexity limits, it could live happily on under $13/month.
In your case if you're able to manage memory carefully you might fit into a 64mb instance and don't require horizontal scaling, you could keep your costs to under $9/month. That seems like it could reasonably be offset through advertising.