Oh, I didn't know someone made such convenient page for average RC costs. I'd normally just look directly at RC stats in log or call rc_api.get_rc_stats. Then again chain is currently so much underused that RC costs are trivial, so it doesn't even make sense to look at them :o)
Every new user is assigned a specific amount of RC (Resource Credits) when registering a new account. This amount is fixed.
It is not changing for account that was created, but it changes in time for different accounts - generally account created later will have slightly smaller RC adjustment value compared to older accounts. It is the value derived from account creation fee by converting it to VESTS at a price from the moment of account creation. Since that price changes, so does the amount of RC. VESTS are gradually getting more expensive (that's how stake holders are getting part of inflation) so the same fee gets you less VESTS later, which means less RC adjustment. Except accounts created before RC was introduced (Hardfork 20) have their RC adjustment fixed with value of 1 HP from the moment of that hardfork, regardless of how much fee went to their creation. Note, it does not matter if the account was created with "free account token" - for the purpose of RC adjustment it counts as full fee from the moment of account creation. Also note that the fee is decided by witnesses, so if they vote to set it to f.e. 30 HIVE instead of 3 like it is today, then accounts created with bigger fee (of token when bigger fee is active) will receive proportionally bigger RC adjustment.
For more technical description of RC I'm going to shamelessly link my old article and also another one about RC stats :o)
Thanks for the feedback. When I was trying to figure out this topic, I spent a few hours trying to find something similar to what you wrote, but I couldn't, so I had to make do with what information I could find.
Thanks to your comment, I now know where I can find a detailed technical description.