The devil is in the details.
A blog doesn’t mean WordPress. I have a blog, for example, and it might look a little like WordPress to the untrained eye, but it’s really all static HTML files created on my local computer by a version of Jekyll called Octopress. That means that all the pages are generated once on my machine, then the results are uploaded to DH. The amount of CPU time it takes to serve a static HTML page is so low that DH doesn’t even measure it in your resource reporting logs.
The number you quote above might be referring to some comments I’ve made about one of my sites. It’s not a blog. 90% of those 250k hits/month, which doesn’t happen every month, are Ajax requests. These do require more CPU time than a static HTML page, but the amount is an order of magnitude less than a singe WordPress page. I’ve also worked very hard to cache enough from the database so that each unique visitor may result in perhaps 3-5 database requests for every 150 page views (which include said Ajax requests). All the information that user may need from the database is requested and cached early on in one connection, unlike WordPress.
Also, my heaviest page results in 20 total requests for images (some sprites) and JS and CSS (minified and combined where possible). Subsequent requests are often just a single request with no further assets needed. The static assets are all on another domain and behind CloudFlare. A single WordPress page can, and often does, result in 60-100 total requests for image-heavy sites or just plane unoptimised sites and are usually served from the same user. So that user could be serving 50+ static files at the same time as it’s requesting data from the database and rendering the page. Do that for two or three unique visitors at a time and you could easily run out of memory (the limit is ~100MB) and/or processes (the limit is 25) and/or concurrent connections (I think the limit is 20).
A wiki can be very resource intensive. Each page may require a database call and if users are editing, I could see how even a single wiki on shared hosting with 2k unique visitors per day could run out of resources.
The short response is: you can’t just install a few one-click apps and get 250k requests / month. It isn’t going to happen. To do that, you’ll need to install your own custom apps, or highly optimised versions of existing apps, and spend a significant amount of time endlessly tweaking and benchmarking as noted here.
For most people, it’s easier, faster, and perhaps more economical to just pay more for more resources.