Here is the simple explanation. If I have a relatively small portion of NXT I will very rarely mine a block. Therefore, for it to be worth my while, the fees have to be extra high to make up for this. So for the NXT system work and maintain decentralization, transaction fees have to be grossly inflated or the network has to rely on the good will of the owners of forging nodes.
So the short short of it is NXT/Peercoin need inflated fees to maintain decentralization.
It depends on a lot of factors.
Renting a VPS costs $3-5/month (that's the total monthly cost of operating a node that you can forge on).
Your profit from forging needs to be above that for forging to be justified.
But since we don't know how much 1 NXT will cost, how many txs the NXT network will handle, we can't calculate how much in fees forging will bring, and how much in fiat that will translate to. It's all guessing at this point, it all depends on demand for crypto currencies from general population. Hence, no analysis is possible with any reliable degree of accuracy.
No, the average transaction fees can easily be obtained given you are skillful enough. Someone posted code to walk the blockchain and find the number of wallets that had forged one block. It would not be hard to take that same code and find the average transaction fee per block found. The average expectation could then easily be found if someone cared enough to go figure it out. Accuracy of the analysis could also be measured if one had the proper math skills.
Your analysis however assumes that the labor involved in the upkeep of a node is 0. Most people value their time to some degree.
I'm not knocking NXT. I think they've done a lot of neat things and are continuing to do neat things. Just because no one cares enough to do the actual full on analysis doesn't make the hypothesis invalid.