Is it cost-effective if you only reload one caliber and you shoot 500 rounds a year?
Probably not, but let's not stop there. If you reload you will get to shoot more because it will be cheaper. I tell people that it's worth reloading if you will shoot more - it's a win/win. Reloading is fun and shooting is fun, so one feeds on the other.
What is required to get into it at an entry level?
For a couple hundred dollars you can start with a Lee single station kit.
Do pistol and rifle cartridges require different presses? i.e. can a basic setup do .22-250 and .40 as long as you have the correct dies and associated parts?
There are a couple progressive presses than may not work with pistol and rifle, but I don't know of any single station presses that won't reload both that you have described.
Wayne