Here’s a handy little chart showcasing how experience gains rise in relation to the Pit tiers!
Icy2889 put together a nice visual representation of both monster kill and Pit finish experience. We can see the rising amounts, but it seems the actual completion XP goes up a lot more than the regular kills.
Icy2889 also went into some optimization, to see which pit tier is the optimal for your particular paragon and gear level. There’s an in-depth explanation below, but the long and short of it is that you want to go for the highest tier possible while aiming at around 90 second Pit clears with what gear and build you have available.
You can check out the full details, including all the math and statistics below: